Vast form of barrels (4)
I believe the answer is:
vats
'barrels' is the definition.
'vast form' is the wordplay.
'form' is an anagram indicator (another form of the letters).
'vast' is an anagram of 'VATS'.
'of' is the link.
(Other definitions for vats that I've seen before include "Tanks" , "Large tubs" , "Large casks" , "Large open vessels for storing liquids" , "Brewing vessels" .)