Pole used in tank which is very big (4)
I believe the answer is:
vast
'very big' is the definition.
'vast' can be an answer for 'big' (similar in meaning). I am not certain of the 'very' bit.
'pole used in tank' is the wordplay.
'pole' becomes 's' (abbreviation for South, as in South Pole).
'used in' is an insertion indicator.
'tank' becomes 'vat' (I've seen this before).
's' inserted within 'vat' is 'VAST'.
'which is' is the link.
Can you help me to learn more?
(Other definitions for vast that I've seen before include "Boundless" , "Huge, enormous" , "Gigantic" , "Astronomical" , "Unusually great" .)