Starts to eat gravy — bangs gong for drink (6)
I believe the answer is:
eggnog
'drink' is the definition.
(eggnog is a kind of drink)
'starts to eat gravy bangs gong' is the wordplay.
'starts to' indicates taking the first letters.
'bangs' indicates an anagram (I've seen 'banging' mean this (the order of letters is banged up or smashed)).
The initial letters of 'eat gravy' is 'eg'.
'gong' anagrammed gives 'gnog'.
'eg'+'gnog'='EGGNOG'
'for' acts as a link.
(Other definitions for eggnog that I've seen before include "Milky winter punch" , "Dairy-based drink" , "Drink similar to advocaat" , "Yellowish mixed alcoholic drink" , "Drink (milk punch)" .)