Big tub filled with sulphur is immense (4)
I believe the answer is:
vast
'immense' is the definition.
(I know that immense can be written as vast)
'big tub filled with sulphur' is the wordplay.
'big tub' becomes 'vat' (I've seen this in another clue).
'filled with' means one lot of letters goes inside another.
'sulphur' becomes 'S' (synonyms).
'vat' placed around 's' is 'VAST'.
'is' acts as a link.
(Other definitions for vast that I've seen before include "Colossal" , "Huge, enormous" , "Extremely large or extensive" , "Of very great extent" , "Unusually great" .)