About to demand a barrel (4)
I believe the answer is:
cask
'a barrel' is the definition.
('cask' can be a synonym of 'barrel')
'about to demand' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'to demand' becomes 'ask' ('ask' can be a synonym of 'demand').
'c'+'ask'='CASK'
(Other definitions for cask that I've seen before include "Drink here" , "Measure" , "Large barrel" , "Liquor container" , "VAT?" .)