Huge supply initially put in tank (4)
I believe the answer is:
vast
'huge' is the definition.
(I've seen this before)
'supply initially put in tank' is the wordplay.
'initially' indicates taking the first letters.
'put in' is an insertion indicator.
'tank' becomes 'vat' (I've seen this before).
The first letter of 'supply' is 's'.
's' inserted inside 'vat' is 'VAST'.
(Other definitions for vast that I've seen before include "Gigantic" , "Huge, enormous" , "Of very great extent" , "Boundless" , "Colossal" .)