Huge vats are assembled (4)
I believe the answer is:
vast
'huge' is the definition.
(I've seen this before)
'vats are assembled' is the wordplay.
'are assembled' is an anagram indicator.
'vats' anagrammed gives 'VAST'.
(Other definitions for vast that I've seen before include "Colossal" , "'Huge, immense (4)'" , "Boundless" , "Astronomical" , "Gigantic" .)