An offence in, say, relaxing (6)
I believe the answer is:
easing
'relaxing' is the definition.
(eg relaxing or easing some pressure)
'an offence in say' is the wordplay.
'an' becomes 'a'.
'offence' becomes 'sin' (a sin is an offence or transgression).
'in' is an insertion indicator.
'say' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'a'+'sin'='asin'
'asin' inserted inside 'eg' is 'EASING'.
(Other definitions for easing that I've seen before include "Calming - relaxation" , "Making lighter" , "comforting" , "Becoming less severe" , "Relieving tension" .)