Powerless detective's exhausted after pinching adult (8)
I believe the answer is:
disabled
'powerless' is the definition.
(thesaurus)
'detective's exhausted after pinching adult' is the wordplay.
'detective's' becomes 'di's' (Detective Inspector abbreviation).
'exhausted' becomes 'bled' (to exhaust something might be to bleed it dry).
'after pinching' means one lot of letters goes inside another.
'adult' becomes 'a' (abbreviation).
'dis'+'bled'='disbled'
'disbled' placed around 'a' is 'DISABLED'.
(Other definitions for disabled that I've seen before include "Not able to work" , "Put out of action" , "Incapacitated" , "Having a physical incapacity caused by injury or disease" .)