Scotsman's up for a swindle? (4)
I believe the answer is:
scam
'for a swindle?' is the definition.
(scam is a kind of swindle)
'scotsman's up' is the wordplay.
'scotsman's' becomes 'mac's'.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'macs' in reverse letter order is 'SCAM'.
(Other definitions for scam that I've seen before include "Swindle (sl.)" , "Screw" , "Dubious operation" , "ploy" , "An informal swindle" .)