Small piece about love seat that’s poisonous (9)
I believe the answer is:
toadstool
'that's poisonous' is the definition.
'small piece about love seat' is the wordplay.
'small piece' becomes 'tad'.
'about' indicates putting letters inside.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'seat' becomes 'stool' (stool is a kind of seat**).
'tad' placed around 'o' is 'toad'.
'toad'+'stool'='TOADSTOOL'
(Other definitions for toadstool that I've seen before include "Fungal growth" , "that could be poisonous" , "Something poisonous" , "Capped fungus" , "Plant akin to mushroom" .)