About to request a barrel (4)
I believe the answer is:
cask
'a barrel' is the definition.
(I know that barrel can be written as cask)
'about to request' is the wordplay.
'about' becomes 'c' (abbreviation for circa).
'to request' becomes 'ask' (to ask is to request something).
'c'+'ask'='CASK'
(Other definitions for cask that I've seen before include "Cylinder for liquid" , "large volume of beer" , "Measure" , "Liquor container" , "Wine barrel" .)