Tie up postman back in bar (7)
I believe the answer is:
taproom
'bar' is the definition.
(I know that bar can be written as taproom)
'tie up postman back' is the wordplay.
'tie up' becomes 'moor' (as in mooring a boat).
'postman' becomes 'pat' (children's TV character Postman Pat).
'back' is a reversal indicator.
'moor'+'pat'='moorpat'
'moorpat' back-to-front is 'TAPROOM'.
'in' acts as a link.
(Other definitions for taproom that I've seen before include "Type of bar" , "Public bar" , "liquor shop" , "Source of beer" , "Hotel bar" .)