Tricked when twin marked ballot paper? (6-7)
I believe the answer is:
double-crossed
'tricked' is the definition.
(to double-cross someone is to betray them)
'twin marked ballot paper?' is the wordplay.
'twin' becomes 'double' (identical twins are each other's doubles).
'marked ballot paper?' becomes 'crossed' (you make a cross on a ballot paper to cast a vote).
'double'+'crossed'='DOUBLE-CROSSED'
'when' acts as a link.
(Other definitions for double-crossed that I've seen before include "Cheated by one's own agent" , "Deceived" .)