A fish I see reported to be a fungus! (6)
I believe the answer is:
agaric
'a fungus' is the definition.
(I know that agaric is a type of basidiomycete)
'a fish i see reported' is the wordplay.
'fish' becomes 'gar' (type of gar).
'reported' shows a homophone (sound like).
'see' is a homophone of 'c'.
'a'+'gar'+'i'+'c'='AGARIC'
'to be' is the link.
(Other definitions for agaric that I've seen before include "Fungus, eg, mushroom" , "fungal growth" , "Fungus of mushroom family" , "Type of mushroom" .)