Compare the sound of fungus and alga (5)
I believe the answer is:
liken
'compare' is the definition.
(I know that compare can be written as liken)
'sound of fungus and alga' is the wordplay.
'sound of' indicates a 'sounds like' (homophone) clue.
'fungus and alga' becomes 'lichen' (composite organism of both).
'lichen' is a homophone of 'LIKEN'.
'the' acts as a link.
(Other definitions for liken that I've seen before include "Point out the resemblance (to)" , "Compare" , "Represent as similar" , "Equate" .)