Loudly compare fungus and alga (6)
I believe the answer is:
lichen
'fungus and alga' is the definition.
(composite organism of both)
'loudly compare' is the wordplay.
'loudly' indicates a 'sounds like' (homophone) clue (when spoken - not silently).
'compare' becomes 'liken' (synonyms).
'liken' is a homophone of 'LICHEN'.
(Other definitions for lichen that I've seen before include "It forms tufts on stones" , "Compound plant of fungus and alga" , "Plant growing on stone" , "It grows on rocks and trees" , "It grows on rocks or bare ground" .)