Boxer's punch that shows something's wrong (5)
I believe the answer is:
cross
I believe this is a double definition.
'boxer's punch' is the first definition.
(type of cross in boxing)
'that shows something's wrong' is the second definition.
(a cross marks something as wrong)
(Other definitions for cross that I've seen before include "Annoyed; hybrid" , "Eg, saltire" , "Mongrel" , "Symbol of Christianity" , "frustrated?" .)