President's bottom, bottom President (5)
I believe the answer is:
Here is my best explanation:
'president' is the definition.
'president's bottom bottom' is the subsidiary indication.
'bottom' indicates one should take the final letters.
'bottom' becomes 'rump' ('rump' can be a synonym of 'bottom'**).
The final letter of 'president' is 't'.
(Other definitions for trump that I've seen before include "take trick", "Outdo (with a card?)", "obstruction gone", "Card of the most powerful suit", "High card", "Powerful playing card", "Powerful card - the last one to be heard at the Day of Judgement", "Valuable card", "A playing card given added value in some games" and "Card of designated top suit".)