One very gluttonous chap drinking squash? On the contrary (8)
I believe the answer is:
gourmand
'one very gluttonous' is the definition.
(a gluttonous person)
'chap drinking squash? on the contrary' is the wordplay.
'chap' becomes 'man' (I've seen this before).
'drinking' indicates putting letters inside.
'squash? on the contrary' becomes 'gourd' (I can't explain this - if you can you should believe this answer much more).
'man' inserted into 'gourd' is 'GOURMAND'.
(Other definitions for gourmand that I've seen before include "Excessive eater and drinker" , "One keen on eating" , "Serious eater" , "One who enjoys eating" , "Epicure" .)