Highest poker cards (4)
I believe the answer is:
aces
'highest poker cards' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for aces that I've seen before include "Honours" , "excellent service" , "that are non-returnable" , "Experts (in their field)" , "Playing cards with single pips on their faces" .)