Amphibian's device to produce fungus (9)
I believe the answer is:
toadstool
'fungus' is the definition.
(I know that toadstool is a type of agaric)
'amphibian's device' is the wordplay.
'amphibian's' becomes 'toad's' (toad is a kind of amphibian).
'device' becomes 'tool' (I've seen this before).
'toads'+'tool'='TOADSTOOL'
'to produce' is the link.
(Other definitions for toadstool that I've seen before include "Something poisonous" , "that could be poisonous" , "what's poisonous maybe" , "Fungal growth" , "Support for little people" .)