Rich cake and drink found in Gaul, mostly (6)
I believe the answer is:
gateau
'rich cake' is the definition.
(I've seen this before)
'drink found in gaul mostly' is the wordplay.
'drink' becomes 'tea' (tea is a kind of drink).
'found in' indicates putting letters inside.
'mostly' means to remove the last letter.
'gaul' with its final letter removed is 'gau'.
'tea' inserted inside 'gau' is 'GATEAU'.
'and' acts as a link.
(Other definitions for gateau that I've seen before include "named for Black Forest?" , "Something French for 22?" , "Cake (from the Black Forest?)" , "Afternoon snack" , "Cake for Pierre" .)