Wound loony up (4)
I believe the answer is:
stab
'wound' is the definition.
(thesaurus)
'loony up' is the wordplay.
'loony' becomes 'bats' ('bats' can be a synonym of 'loony').
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'bats' reversed gives 'STAB'.
(Other definitions for stab that I've seen before include "It could be the death of you" , "Twinge; guess" , "Thrust at and pierce" , "Strike with knife" , "Wound with blade" .)