Huge tank storing sulphur (4)
I believe the answer is:
vast
'huge' is the definition.
(thesaurus)
'tank storing sulphur' is the wordplay.
'tank' becomes 'vat' (I've seen this before).
'storing' is an insertion indicator.
'sulphur' becomes 's' ('S' can be a synonym of 'sulphur').
'vat' going around 's' is 'VAST'.
(Other definitions for vast that I've seen before include "Of very great extent" , "Gigantic" , "Astronomical" , "'Huge, immense (4)'" , "Unusually great" .)