Huge barrel containing sulphur (4)
I believe the answer is:
vast
'huge' is the definition.
(I know that huge can be written as vast)
'barrel containing sulphur' is the wordplay.
'barrel' becomes 'vat' (a vat is a large tub or barrel).
'containing' indicates putting letters inside.
'sulphur' becomes 'S' (synonyms).
'vat' enclosing 's' is 'VAST'.
(Other definitions for vast that I've seen before include "Unusually great" , "Gigantic" , "Extremely large or extensive" , "Of very great extent" , "Astronomical" .)