Large vat (3)
I believe the answer is:
tun
'large vat' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tun that I've seen before include "vessel" , "Large cask or barrel" , "Nut found in large cask" , "Cask, vat" , "Large cask for liquid" .)