Secreted chemical: call to inject extra rejected (9)
I believe the answer is:
pheromone
'secreted chemical' is the definition.
'call to inject extra rejected' is the wordplay.
'call' becomes 'phone' (synonyms).
'to inject' means one lot of letters goes inside another (inserted letters are injected).
'extra' becomes 'more' (both can mean additional).
'rejected' says the letters should be written in reverse.
'more' backwards is 'erom'.
'phone' going around 'erom' is 'PHEROMONE'.
(Other definitions for pheromone that I've seen before include "Chemical attractor" , "Substance secreted that affects others of same species" , "Chemical signal between organisms" , "Chemical affecting behaviour" , "What attracts" .)