Substitutes tolerate offence (6,2)
I believe the answer is:
stands in
'substitutes' is the definition.
('stand in' can be a synonym of 'substitute')
'tolerate offence' is the wordplay.
'tolerate' becomes 'stand' (synonyms).
'offence' becomes 'sin' (a sin is an offence or transgression).
'stand'+'sin'='STANDS IN'
(Other definitions for stands in that I've seen before include "Acts as sub" , "Substitutes" , "acts for the moment" , "Acts as a substitute" , "Deputises" .)