Experts: card sharks, primarily (4)
I believe the answer is:
aces
'experts' is the definition.
(ace is a kind of expert)
'card sharks primarily' is the wordplay.
'card' becomes 'ace' (ace is a kind of card).
'primarily' suggests taking the first letters.
The initial letter of 'sharks' is 's'.
'ace'+'s'='ACES'
(Other definitions for aces that I've seen before include "winning shots" , "Case of top cards" , "never come back" , "Tennis serves the receiver can't reach" , "wartime pilots?" .)