Dishonest man on board in last month returned (7)
I believe the answer is:
crooked
'dishonest' is the definition.
('crooked' can be similar in meaning to 'dishonest')
'man on board in last month returned' is the wordplay.
'man on board' becomes 'rook' (chess).
'in' means one lot of letters goes inside another.
'last month' becomes 'dec'.
'returned' is a reversal indicator.
'dec' back-to-front is 'ced'.
'rook' put within 'ced' is 'CROOKED'.
(Other definitions for crooked that I've seen before include "Bent; dishonest" , "Distorted or dishonest" , "Not straight or aligned" , "Warped" , "Awry - untrustworthy" .)