Experts at 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)
'at' acts as a link.
(Other definitions for aces that I've seen before include "winning points" , "passes with flying colours in US" , "Champions" , "Experts (in their field)" , "They're high and low on cards" .)