Only time for cask (4)
I believe the answer is:
butt
'for cask' is the definition.
(butt is a kind of cask)
'only time' is the wordplay.
'only' becomes 'but' (but can mean only - eg 'life is but a dream').
'time' becomes 't' (abbreviation).
'but'+'t'='BUTT'
(Other definitions for butt that I've seen before include "couple of hogsheads?" , "Knock head" , "Scapegoat - strike - bottom - barrel" , "Stub" , "Bottom end of a rifle" .)