Back in flat, celebrating return (5)
I believe the answer is:
elect
'return' is the definition.
(I know that return is a more specific form of the action elect)
'back in flat celebrating' is the wordplay.
'back' is a reversal indicator.
'in' indicates a hidden word.
'tcele' is hidden in the letters of 'flat celebrating'.
'tcele' backwards is 'ELECT'.
(Other definitions for elect that I've seen before include "Appoint through ballot" , "Put in power by voting" , "Waiting to get into office" , "Vote into office" , "Put into power through the ballot box" .)