Fraud fishy people must use (4)
I believe the answer is:
hype
'fraud' is the definition.
(hype can mean fraudulently promoting something)
'fishy people must use' is the wordplay.
'use' indicates a hidden word (I've seen 'using' mean this (the answer is used in part of the clue)).
'HYPE' is hidden within 'fishy people must'.
(Other definitions for hype that I've seen before include "Exaggerated advocacy" , "used by pusher?" , "Publicity stunt" , "Intensive promotion or excitement" , "Promotional publicity" .)