Starting to pester a paper concerning a diamond (7)
I believe the answer is:
paragon
'a diamond' is the definition.
(I know that paragon is a diamond)
'starting to pester a paper concerning' is the wordplay.
'starting to' says to take the initial letters.
'paper' becomes 'rag' (rag can mean a newspaper).
'concerning' becomes 'on' (both can mean 'in relation to').
The initial letter of 'pester' is 'p'.
'p'+'a'+'rag'+'on'='PARAGON'
(Other definitions for paragon that I've seen before include "Model of excellence or perfection" , "Exemplar" , "A model as of virtue" , "A perfect person - pagan or what?" , "Perfect example of a quality" .)