Offence in losing (3)
I believe the answer is:
sin
'offence' is the definition.
(a sin is an offence or transgression)
'in losing' is the wordplay.
'in' says the answer is hidden in the clue.
'SIN' is hidden within 'losing'.
(Other definitions for sin that I've seen before include "Wrongdoing" , "Breach of moral code" , "'Transgression, mortal or venial (3)'" , "Go astray" , "Transgression of moral law" .)