Work together sealing politician's brief (7)
I believe the answer is:
compact
'brief' is the definition.
(I've seen this before)
'work together sealing politician's' is the wordplay.
'work together' becomes 'coact' (I've seen this in another clue).
'sealing' indicates putting letters inside (inserted letters are sealed inside).
'politician' becomes 'MP' (Member of Parliament).
'coact' enclosing 'mp' is 'COMPACT'.
(Other definitions for compact that I've seen before include "Tightly grouped" , "it could be in the bag" , "Dense; agreement" , "Small case for face powder" , "Dense; treaty" .)