Returning alumnae consume bottles -- a vast quantity (5)
I believe the answer is:
ocean
'a vast quantity' is the definition.
(I've seen this before)
'returning alumnae consume bottles' is the wordplay.
'returning' shows that the letters should be reversed in order.
'bottles' indicates the answer is hidden within the clue (bottling is a kind of containing).
'naeco' is hidden within 'alumnae consume'.
'naeco' written backwards gives 'OCEAN'.
(Other definitions for ocean that I've seen before include "eg Indian, Pacific" , "Large stretch of water" , "One of the Seven Seas" , "Large stretch of sea" , "Sea - canoe (anag)" .)