They're dab hands at cards (4)
I believe the answer is:
aces
'cards' is the definition.
(ace is a kind of card)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for aces that I've seen before include "Tennis serves the receiver can't reach" , "winning shots" , "Case of top cards" , "passes with flying colours in US" , "They're high and low on cards" .)