Former spouse to lie about banishment (5)
I believe the answer is:
exile
'banishment' is the definition.
(exile is a kind of banishment)
'former spouse to lie about' is the wordplay.
'former spouse' becomes 'ex' (an ex-spouse).
'about' indicates anagramming the letters.
'lie' is an anagram of 'ile'.
'ex'+'ile'='EXILE'
(Other definitions for exile that I've seen before include "Person absent from home or country" , "forced to leave" , "Overseas banishment" , "Person who now lives in another country" , "Drive away" .)