Huge vats are used (4)
I believe the answer is:
vast
'huge' is the definition.
(I've seen this before)
'vats are used' is the wordplay.
'are used' indicates anagramming the letters.
'vats' with letters rearranged gives 'VAST'.
(Other definitions for vast that I've seen before include "'Huge, immense (4)'" , "Unusually great" , "Astronomical" , "Gigantic" , "Colossal" .)