Particle put forward and not rebounding (8)
I believe the answer is:
positron
'particle' is the definition.
(positron is a kind of particle)
'put forward and not rebounding' is the wordplay.
'put forward' becomes 'posit' (to posit an idea is to put it forward).
'and not' becomes 'nor' ('nor' means 'and also not').
'rebounding' says the letters should be written backwards (I've seen 'rebounds' mean this).
'nor' backwards is 'ron'.
'posit'+'ron'='POSITRON'
(Other definitions for positron that I've seen before include "Charged particle" , "Sub-atomic particle" , "Elementary particle with positive charge" , "possible contributor to annihilation" , "there's a small charge here" .)