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