Catch adult wearing nothing (4)
I believe the answer is:
nail
'catch' is the definition.
(nail can mean to catch or arrest)
'adult wearing nothing' is the wordplay.
'adult' becomes 'a' (abbreviation).
'wearing' indicates putting letters inside.
'nothing' becomes 'nil' (eg nil is zero in football).
'a' going into 'nil' is 'NAIL'.
(Other definitions for nail that I've seen before include "Apprehend" , "Hit this on the head to be bang on" , "Hits this in the head to be bang-on" , "Metal fastener" , "A small steel wood pin" .)