Amphibian seen on seat with fungus (9)
I believe the answer is:
toadstool
'fungus' is the definition.
(toadstool is a kind of fungus)
'amphibian seen on seat' is the wordplay.
'amphibian' becomes 'toad' (toad is a kind of amphibian).
'seen on' says to put letters next to each other.
'seat' becomes 'stool' (stool is a kind of seat).
'toad'+'stool'='TOADSTOOL'
'with' is the link.
(Other definitions for toadstool that I've seen before include "Poisonous fungus" , "Common fungus" , "Something poisonous" , "what's poisonous maybe" , "Support for little people" .)