Company gets seaweed and sulphur as fuel (4-3)

'fuel' is the definition.
(coal gas is a kind of fuel)

'company gets seaweed and sulphur' is the wordplay.
'company' becomes 'co' (abbreviation).
'gets' is a charade indicator (letters next to each other).
'seaweed' becomes 'alga' (singular form of 'algae').
'and' is a charade indicator (letters next to each other).
'sulphur' becomes 'S' (synonyms).
'co'+'alga'+'s'='COAL-GAS'

'as' is the link.