Bar offers beer, about a litre (5)
I believe the answer is:
block
'bar' is the definition.
('block' can be a synonym of 'bar')
'beer about a litre' is the wordplay.
'beer' becomes 'bock' (bock is a kind of beer).
'about' is an insertion indicator.
'a litre' becomes 'l' ('l' can be a synonym of 'litre').
'bock' going around 'l' is 'BLOCK'.
'offers' is the link.
(Other definitions for block that I've seen before include "Mass of wood etc - set of buildings" , "Solid lump" , "Obstruct" , "cake?" , "Solid shape; prevent" .)