Powerful playing card (5)
I believe the answer is:
trump
'powerful playing card' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for trump that I've seen before include "US President" , "Card of designated top suit" , "Powerful card - the last one to be heard at the Day of Judgement" , "High card" , "The last one to announce" .)