Outing former lover for putting on an act (8)
I believe the answer is:
exposing
'outing' is the definition.
(I've seen this before)
'former lover for putting on an act' is the wordplay.
'former lover' becomes 'ex' (an ex-lover).
'for' means one lot of letters go next to another.
'putting on an act' becomes 'posing' (I've seen this before).
'ex'+'posing'='EXPOSING'
(Other definitions for exposing that I've seen before include "outing" , "Showing up" , "Subjecting to light, making known" , "Uncovering" .)