Massive vats are used (4)
I believe the answer is:
vast
'massive' is the definition.
(thesaurus)
'vats are used' is the wordplay.
'are used' indicates anagramming the letters.
'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" .)