Relatives firm with us about moral offence (7)
I believe the answer is:
cousins
'relatives' is the definition.
(I know this)
'firm with us about moral offence' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'with' is a charade indicator (letters next to each other).
'about' indicates putting letters inside.
'moral offence' becomes 'sin' (I've seen this before).
'us' going around 'sin' is 'usins'.
'co'+'usins'='COUSINS'
(Other definitions for cousins that I've seen before include "Relatives" , "kinsmen" .)