Betray guerrilla fighter, then almost make amends (5,2)

'betray' is the definition.
(synonyms)

'guerrilla fighter then almost make amends' is the wordplay.
'guerrilla fighter' becomes 'Che' (revolutionary Che Guevara).
'then' is a charade indicator (letters next to each other).
'almost' means to remove the last letter (most of the word but not all of it).
'make amends' becomes 'atone' (to atone is to make amends for a wrong deed).
'atone' with its final letter taken away is 'aton'.
'che'+'aton'='CHEAT ON'