Behind POTUS? Tense doesn’t come into it! (4)

I believe the answer is:

rump

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'behind' is the definition.
(informal term for the buttocks)

I cannot understand how the rest of the clue works.

(Other definitions for rump that I've seen before include "Inferior remains; cut of steak" , "Tail end" , "Rear; buttocks" , "Seat" , "Cut of meat form animal's hindquarters" .)

I've seen this clue in The Times.

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