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 a hidden word.
'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" .)