The way the clearance works is when you get out it is deactivated. So no, you don't have an active clearance, even if you go into the IRR. The investigation is good for five years and if the DoD or one of their contractors hire you, they can simply submit paperwork to reactivate the clearance or request a lower one.
If you go to another department within the government, they have to request the investigation and if you are suitable they can request the clearance be activated. This is where the LE side and the classified side clash. There are different adjudication standards for each. Theoretically they should be able to order the investigation and submit for a new clearance, but often the LE agency will have more questions to ask that will make the old investigation unusable. The time lag between requesting the investigation and receiving it can be long, longer than actually doing the new investigation. Your old investigation will be ordered and reviewed for you new investigation. While the agency is waiting for the old investigation they can be conducting your new one and provide you with an interim clearance and put you to work. If they waited for your old investigation you and the new agency can be waiting several months longer to go to work. So even though you have the clearance and the valid investigation you will probably have to undergo another one for the Fed LE job.
On a side note, there are plenty of Fed LE jobs that will stop your 12d retirement clock (ICE, DEA, ATF, CBPO, Border Patrol, BOP, some Park Rangers, I think Capitol Police, etc.). These jobs will give you the same opportunity to obtain your degree as a local agency and give you more time toward 12d retirement. The reason I bring this up is sometimes life gets in the way and you are unable to go to school for a few semesters after you start. In LE sometimes it is difficult to go to school. Figure it will take you at least a year, often longer, to get the job you want after you apply. The federal government is notorious for having hiring freezes. Say it takes you 6 or 7 years to finish your degree. With all of these things in consideration, if you are at least 27 years old, you might either just get in under the wire or you will be too old and no longer eligible. If you get one of these federal jobs covered by 12d retirement, then it wouldn't matter if your over the age limit when you apply at your dream job because as far at the government is concerned you stopped aging when you were covered by 12d. Federal civil service is good, very good. You can change jobs often and never lose your retirement. So if you get a job with Capitol Police and decide LE isn't for you, you can always get a job as a "grease monkey" as you put it and still keep your retirement. One of my co-workers changes jobs about every three or four years, all of them are federal jobs so he never loses retirement time.
But when a long train of abuses and usurpations, pursuing invariably the same object, evinces a design to reduce them under absolute Despotism, it is their right, it is their duty, to throw off such Government, and to provide new guards for their future security.
Translation for the intellectually challenged: If the government screws the people too much, it is the right and duty of the people to revolt and form a new government.