Drink that is a snack (7)
I believe the answer is:
toastie
'a snack' is the definition.
(a toasted snack)
'drink that is' is the wordplay.
'drink' becomes 'toast' (toast is a kind of drink).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'toast'+'ie'='TOASTIE'
(Other definitions for toastie that I've seen before include "Warm snack" , "Browned sandwich" , "Grilled food" , "Toasted sandwich" , "Hot sandwich" .)