Initially strike over type of fuel, it's assumed (9)
I believe the answer is:
sobriquet
'it's assumed' is the definition.
(a sobriquet might be an assumed name)
'initially strike over type of fuel' is the wordplay.
'initially' says to take the initial letters.
'type of fuel' becomes 'briquet' (block of coal-dust or charcoal).
The initial letters of 'strike over' is 'so'.
'so'+'briquet'='SOBRIQUET'
(Other definitions for sobriquet that I've seen before include "Person's nickname" , "dubious name" , "Another name" , "one's assumed" , "Nickname -- Boers quit (anag)" .)