Tricky! I can get to answers through guesswork and some parts of the clue, but some help with parsing would be incredibly welcome
1ac One making prisoner fitter: Conformer?
15ac Not quite blue, my new fur: Raccoon?
18ac Got around anything? : Caught?
5dn From cavity - it's morning! (Apparently) : Sinus?
13dn Circular pipe breaks while creating pools of water? Billabongs?