Seaweed in lettuce? That will cause a stink (4,3)
I believe the answer is:
coal gas
'that will cause a stink' is the definition.
(gas with a strong smell)
'seaweed in lettuce?' is the wordplay.
'seaweed' becomes 'alga' (I've seen this before).
'in' means one lot of letters goes inside another.
'lettuce?' becomes 'cos' (cos is a kind of lettuce).
'alga' put into 'cos' is 'COAL-GAS'.
(Other definitions for coal gas that I've seen before include "Heating and lighting fuel" , "Source of power" , "Former fuel" .)