Experts in cards (4)
I believe the answer is:
aces
I believe this clue is a double definition.
'experts' is the first definition.
(ace is a kind of expert)
'cards' is the second definition.
(ace is a kind of card)
'in' acts as a link.
(Other definitions for aces that I've seen before include "Case of top cards" , "Top cards or people" , "Flying war heroes" , "that are non-returnable" , "Two will make deuce" .)