Excused former partner, mostly insincere (6)
I believe the answer is:
exempt
'excused' is the definition.
(similar in meaning)
'former partner mostly insincere' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'mostly' means to remove the last letter (most but not all of the word).
'insincere' becomes 'empty' (I've seen this before).
'empty' with its final letter taken off is 'empt'.
'ex'+'empt'='EXEMPT'
(Other definitions for exempt that I've seen before include "Not subject to (eg taxation)" , "Free from, eg, tax" , "Immune - absolve" , "Absolved" , "Not liable to, immune" .)