Swindle sees rascal ultimately wanting (4)
I believe the answer is:
scam
'swindle' is the definition.
(I know that scam is a type of swindle)
'rascal ultimately wanting' is the wordplay.
'rascal' becomes 'scamp' (synonyms).
'ultimately wanting' means to remove the last letter (wanting can mean lacking).
'scamp' with its last letter taken off is 'SCAM'.
'sees' acts as a link.
(Other definitions for scam that I've seen before include "A slangy swindle or fraud" , "Rip-off" , "Fraud, dishonest scheme" , "Dubious operation" , "Racket" .)