Top playing cards (4)
I believe the answer is:
aces
'top playing cards' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aces that I've seen before include "that are non-returnable" , "They're high and low on cards" , "Unreturnable services" , "excellent service" , "Ones or elevens in cards" .)