Errant mate returned to confess (7)
I believe the answer is:
lapsing
'errant' is the definition.
(both can mean slipping or straying)
'mate returned to confess' is the wordplay.
'mate' becomes 'pal' (I've seen this before).
'returned' is a reversal indicator.
'to confess' becomes 'sing' (I've seen this before).
'pal' written backwards gives 'lap'.
'lap'+'sing'='LAPSING'
(Other definitions for lapsing that I've seen before include "Reverting" , "Backsliding" .)