In advance, post offices put together (7)
I believe the answer is:
compose
'put together' is the definition.
(synonyms)
'in advance post offices' is the wordplay.
'in' means one lot of letters goes inside another.
'advance' becomes 'come' (I've seen this in other clues).
'post offices' becomes 'pos' (PO is an abbreviation for post office).
'come' enclosing 'pos' is 'COMPOSE'.
(Other definitions for compose that I've seen before include "Write a piece of music" , "TRITE/WRITE" , "Devise" , "18 [GET READY]" , "Settle - score" .)