Bigger beer round back of bar (6)
I believe the answer is:
larger
'bigger' is the definition.
('large' can be a synonym of 'big')
'beer round back of bar' is the wordplay.
'beer' becomes 'lager' (lager is a kind of beer).
'round' is an insertion indicator.
'back of' suggests the final letters.
The final letter of 'bar' is 'r'.
'lager' going around 'r' is 'LARGER'.
(Other definitions for larger that I've seen before include "Comparatively bigger" , "Greater in size" , "Ebullient person may be describes as ?? than life" , "Bigger, greater" , "More bulky" .)