Fishy character suppressing yen to attempt scam (3,2,2)
I believe the answer is:
try it on
'to attempt scam' is the definition.
'fishy character suppressing yen' is the wordplay.
'fishy character' becomes 'triton' (Greek sea god depicted as merman).
'suppressing' is an insertion indicator.
'yen' becomes 'y'.
'triton' going around 'y' is 'TRY IT ON'.
(Other definitions for try it on that I've seen before include "Attempt to get away with something" , "Before buying a garment, you should" , "trick" , "be bold" , "practise a little deception" .)