The ones in hand (4)
I believe the answer is:
aces
'the ones' is the definition.
('ace' can be a synonym of 'one')
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for aces that I've seen before include "Unreturnable services" , "winning points" , "Two will make deuce" , "Top cards or people" , "Experts (in their field)" .)