Huge vats, possibly (4)
I believe the answer is:
vast
'huge' is the definition.
(I've seen this before)
'vats possibly' is the wordplay.
'possibly' indicates an anagram.
'vats' is an anagram of 'VAST'.
(Other definitions for vast that I've seen before include "Gigantic" , "Astronomical" , "'Huge, immense (4)'" , "Of very great extent" , "Huge, enormous" .)