Massive vats are used (4)
I believe the answer is:
vast
'massive' is the definition.
(thesaurus)
'vats are used' is the wordplay.
'are used' is an anagram indicator.
'vats' anagrammed gives 'VAST'.
(Other definitions for vast that I've seen before include "Extremely large or extensive" , "Unusually great" , "Of very great extent" , "'Huge, immense (4)'" , "Astronomical" .)