Scottish island conceals a new fuel (6)
I believe the answer is:
butane
'fuel' is the definition.
(butane is a kind of fuel)
'scottish island conceals a new' is the wordplay.
'scottish island' becomes 'bute' (I've seen this in other clues).
'conceals' indicates putting letters inside.
'a new' becomes 'an' (I've seen this in other clues).
'bute' enclosing 'an' is 'BUTANE'.
(Other definitions for butane that I've seen before include "Flammable gas in petroleum" , "Bottled gas used as a fuel" , "Lighter fuel" , "Inflammable gas produced from petrol" .)