Massive container holding sulphur (4)
I believe the answer is:
vast
'massive' is the definition.
(thesaurus)
'container holding sulphur' is the wordplay.
'container' becomes 'vat' (vat is a kind of container).
'holding' 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" , "Huge, enormous" , "Boundless" , "Astronomical" , "'Huge, immense (4)'" .)