Firms gathering seaweed for fuel (4,3)
I believe the answer is:
coal gas
'fuel' is the definition.
(coal gas is a kind of fuel)
'firms gathering seaweed' is the wordplay.
'firms' becomes 'cos' (co is short for company - a business or firm).
'gathering' is an insertion indicator.
'seaweed' becomes 'alga' (I've seen this before).
'cos' going around 'alga' is 'COAL-GAS'.
'for' is the link.
(Other definitions for coal gas that I've seen before include "Heating and lighting fuel" , "Former fuel" , "Source of power" .)