King in French resort left US coin (6)
I believe the answer is:
nickel
'us coin' is the definition.
(five cent coin in the US)
'king in french resort left' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'in' means one lot of letters goes inside another.
'french resort' becomes 'nice' (French coastal resort).
'left' becomes 'l' (common abbreviation).
'k' going inside 'nice' is 'nicke'.
'nicke'+'l'='NICKEL'
(Other definitions for nickel that I've seen before include "Silvery-white metal" , "One twentieth of a US dollar" , "Metal; US_coin" , "Metal - US coin" , "Sliver of wood/glass" .)