Huge vats are used (4)
I believe the answer is:
vast
'huge' is the definition.
(I know that huge can be written as vast)
'vats are used' is the wordplay.
'are used' indicates anagramming the letters.
'vats' is an anagram of 'VAST'.
(Other definitions for vast that I've seen before include "'Huge, immense (4)'" , "Unusually great" , "Astronomical" , "Gigantic" , "Colossal" .)