Fuel from seaweed wrapped in lettuce (4,3)
I believe the answer is:
coal gas
'fuel' is the definition.
(coal gas is a kind of fuel)
'seaweed wrapped in lettuce' is the wordplay.
'seaweed' becomes 'alga' (I've seen this before).
'wrapped in' is an insertion indicator.
'lettuce' becomes 'cos' (cos lettuce is a kind of lettuce).
'alga' going into 'cos' is 'COAL-GAS'.
'from' is the link.
(Other definitions for coal gas that I've seen before include "Former fuel" , "Source of power" , "Heating and lighting fuel" .)