A little matter for a professional heavyweight (6)
I believe the answer is:
proton
'a little matter' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'professional heavyweight' is the wordplay.
'professional' becomes 'pro' (abbreviation).
'heavyweight' becomes 'ton' (a ton weighs a lot).
'pro'+'ton'='PROTON'
'for a' is the link.
Can you help me to learn more?
(Other definitions for proton that I've seen before include "just a little bit" , "Tiny item charged" , "Elementary particle found in all atomic nuclei" , "Positive-charged particle" , "Stable subatomic particle" .)