Stuff you need before filling old tank (7)
I believe the answer is:
overeat
'stuff' is the definition.
('overeat' can be a synonym of 'stuff')
'before filling old tank' is the wordplay.
'before' becomes 'ere' (usually literary).
'filling' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'tank' becomes 'vat' (I've seen this before).
'o'+'vat'='ovat'
'ere' inserted into 'ovat' is 'OVEREAT'.
'you need' acts as a link.
(Other definitions for overeat that I've seen before include "Gorge oneself" , "Indulge in gluttony" , "Gobble to excess" , "Be greedy" , "Stuff oneself silly" .)