Drink beer on me, possibly after end of poll (6,5)
I believe the answer is:
Here is my best explanation:
'drink' is the definition.
(bitter lemon is a kind of drink)
'beer on me possibly after end of poll' is the subsidiary indication.
'beer' becomes 'bitter' (I've seen this before**).
'possibly' indicates anagramming the letters.
'after' means one lot of letters go next to another.
'end of' suggests the final letters.
The last letter of 'poll' is 'l'.
'onme' with letters rearranged gives 'emon'.
'emon' put after 'l' is 'lemon'.
(Other definitions for bitter lemon that I've seen before include "Soft citrus drink", "A soft drink" and "Tart soft drink".)