Former lover acting falsely is revealing (8)
I believe the answer is:
exposing
'revealing' is the definition.
('expose' can be a synonym of 'reveal')
'former lover acting falsely' is the wordplay.
'former lover' becomes 'ex' (ex-lover).
'acting falsely' becomes 'posing' (I can't justify this - if you can you should give a lot more credence to this answer).
'ex'+'posing'='EXPOSING'
'is' is the link.
Can you help me to learn more?
(Other definitions for exposing that I've seen before include "outing" , "Showing up" , "Subjecting to light, making known" , "Uncovering" .)