Fungus frog-like creatures also left (9)
I believe the answer is:
toadstool
'fungus' is the definition.
(toadstool is a kind of fungus)
'frog-like creatures also left' is the wordplay.
'frog-like creatures' becomes 'toads'.
'also' becomes 'too' ('too' can be a synonym of 'also').
'left' becomes 'l' (common abbreviation).
'toads'+'too'+'l'='TOADSTOOL'
(Other definitions for toadstool that I've seen before include "Something poisonous" , "that could be poisonous" , "Umbrella-shaped cap on a stalk" , "a potentially toxic development" , "Poisonous fungus" .)