King's instrument for fiddler? (4,5)
I believe the answer is:
card sharp
'fiddler?' is the definition.
(type of fraud or fiddler)
'king's instrument' is the wordplay.
'king's' becomes 'card's' (king is a type of playing card).
'instrument' becomes 'harp' (harp is a kind of instrument).
'cards'+'harp'='CARD-SHARP'
'for' acts as a link.
(Other definitions for card sharp that I've seen before include "Cheating gambler" , "cheat in casino" , "Trickster" , "You won't get a good deal out of one" , "one whose hands are not to be trusted" .)