Tiny thing to put forward and not back (8)
I believe the answer is:
positron
'tiny thing' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'put forward and not back' 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').
'back' shows that the letters should be reversed in order.
'nor' written backwards gives 'ron'.
'posit'+'ron'='POSITRON'
'to' is the link.
Can you help me to learn more?
(Other definitions for positron that I've seen before include "there's a small charge here" , "Elementary particle with positive charge" , "possible contributor to annihilation" , "Sub-atomic particle" , "A tiny thing" .)