To a professional heavyweight, it's almost nothing! (6)
I believe the answer is:
proton
'nothing' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'to a professional heavyweight it's almost' is the wordplay.
'to a professional' becomes 'pro' (abbreviation).
'heavyweight' becomes 'ton' (a ton weighs a lot).
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'almost' means to remove the last letter (most of the word but not all of it).
'pro'+'ton'+'t'='protont'
'protont' with its last letter taken away is 'PROTON'.
Can you help me to learn more?
(Other definitions for proton that I've seen before include "Atomic constituent" , "Positive-charged particle" , "Stable subatomic particle" , "just a little bit" , "Positively charged particle" .)