Drinking a lot of best gin cocktail (6)
I believe the answer is:
toping
'drinking' is the definition.
('tope' can be a synonym of 'drink')
'a lot of best gin cocktail' is the wordplay.
'a lot of' means to remove the last letter (most of the word but not all of it).
'best' becomes 'tops' (I've seen this in another clue).
'cocktail' indicates anagramming the letters.
'tops' with its final letter taken off is 'top'.
'gin' with letters rearranged gives 'ing'.
'top'+'ing'='TOPING'
(Other definitions for toping that I've seen before include "boozing" , "going on regular binges" .)