Large politician surrounded by drink (5)
I believe the answer is:
ample
'large' is the definition.
('ample' can be similar in meaning to 'large')
'politician surrounded by drink' is the wordplay.
'politician' becomes 'mp'.
'surrounded by' means one lot of letters goes inside another.
'drink' becomes 'ale' (ale is a kind of drink).
'mp' placed into 'ale' is 'AMPLE'.
(Other definitions for ample that I've seen before include "Large, capacious" , "No more, please!" , "Sealed glass container for dose of medicine" , "Buxom" , "Fully enough" .)