Instrument used by cardsharpers (4)
I believe the answer is:
harp
'instrument' is the definition.
(I know that Harp is a instrument)
'used by cardsharpers' is the wordplay.
'used by' indicates a hidden word.
'HARP' can be found hidden inside 'cardsharpers'.
(Other definitions for harp that I've seen before include "Emblem of Ireland" , "Musical symbol" , "''The ... that once through Tara's halls'' (4)" , "Instrument for angel" , "semitone higher, abandoning intro" .)