Offence of losing (3)
I believe the answer is:
sin
'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' can be found hidden inside 'losing'.
(Other definitions for sin that I've seen before include "Depravity" , "Pride, say" , "Fall from grace" , "Mortal error" , "Moral offence" .)