It's very small and in favour of putting on weight (6)
I believe the answer is:
proton
'it's very small' is the definition.
I can't judge whether this definition defines the answer.
'in favour of putting on weight' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'putting on' says to put letters next to each other.
'weight' becomes 'ton' (unit of weight).
'pro'+'ton'='PROTON'
'and' is the link.
Can you help me to learn more?
(Other definitions for proton that I've seen before include "just a little bit" , "One's charged" , "Particle in atom nucleus" , "Positively charged particle" , "Positive-charged particle" .)