Is one on the fiddle in card game? (6)
I believe the answer is:
bridge
This clue is a double definition.
'is one on the fiddle' is the first definition.
(a violin or fiddle has a bridge)
'card game?' is the second definition.
(I know that bridge is a type of card game)
'in' acts as a link.
(Other definitions for bridge that I've seen before include "spanner" , "Hard ridge on the nose" , "indoor game" , "Where the ship is steered and the captain stands" , "may involve suspension" .)