Unmask former partner with model (6)
I believe the answer is:
expose
'unmask' is the definition.
(I know that expose is a is a more specific form of the action of unmask)
'former partner with model' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'with' means one lot of letters go next to another.
'model' becomes 'pose' (model can mean to pose).
'ex'+'pose'='EXPOSE'
(Other definitions for expose that I've seen before include "hazard" , "Unmask or denounce" , "outing" , "Bring to light" , "Disclose" .)