About to demand a strong wooden barrel (4)
I believe the answer is:
cask
'a strong wooden barrel' is the definition.
'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" , "Keg" , "Wine vessel; sack (anag.)" , "Cylinder for liquid" , "Liquor container" .)