To notice seat and growth (9)
I believe the answer is:
toadstool
'growth' is the definition.
(I have seen 'Fungal growth' mean 'toadstool' so perhaps 'growth' could also mean 'toadstool')
'to notice seat' is the wordplay.
'notice' becomes 'ad' (short for advert).
'seat' becomes 'stool' (stool is a kind of seat).
'to'+'ad'+'stool'='TOADSTOOL'
'and' is the link.
(Other definitions for toadstool that I've seen before include "Umbrella-shaped fungus" , "Something poisonous" , "Umbrella-shaped cap on a stalk" , "that could be poisonous" , "Fungal growth" .)