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' indicates the answer is hidden within the clue.
'SIN' is hidden in the letters of 'losing'.
(Other definitions for sin that I've seen before include "Depravity" , "Pride, say" , "Fall from grace" , "Mortal error" , "Moral offence" .)