Positive thing in favour of weight (6)
I believe the answer is:
proton
'positive thing' is the definition.
(I've seen this before)
'in favour of weight' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'weight' becomes 'ton' (unit of weight).
'pro'+'ton'='PROTON'
(Other definitions for proton that I've seen before include "One charged" , "Positively charged particle" , "Stable subatomic particle" , "Particle in atoms" , "Positive nucleon" .)