The offence of losing (3)
I believe the answer is:
sin
'the offence' is the definition.
(a sin is an offence or transgression)
'of losing' is the wordplay.
'of' says the answer is hidden in the clue.
'SIN' is hidden in the letters of 'losing'.
(Other definitions for sin that I've seen before include "Wickedness" , "Immoral action" , "Breach of moral law" , "Misdeed" , "Immoral act" .)