Liberal politician into the drink (5)
I believe the answer is:
ample
'liberal' is the definition.
(I've seen this in another clue)
'politician into the drink' is the wordplay.
'politician' becomes 'MP' (Member of Parliament).
'into' means one lot of letters goes inside another.
'the drink' becomes 'ale' (type of beer).
'mp' going inside 'ale' is 'AMPLE'.
(Other definitions for ample that I've seen before include "Plentiful; capacious" , "Full-figured" , "Large, capacious" , "Generous" , "No more, please!" .)