Provide sheet to go under drink (6)
I believe the answer is:
supply
'provide' is the definition.
(I know that provide can be written as supply)
'sheet to go under drink' is the wordplay.
'sheet' becomes 'ply' (ply is a kind of sheet).
'to go under' says to put letters next to each other (in a down clue, some letters go below others).
'drink' becomes 'sup' (to sup is to drink).
'ply' put after 'sup' is 'SUPPLY'.
(Other definitions for supply that I've seen before include "Make available" , "Provide or furnish with" , "Fund" , "Provide; flexibly" , "Stock" .)