One small particle? In favour of 100 (6)
I believe the answer is:
proton
'one small particle?' is the definition.
(type of subatomic particle)
'in favour of 100' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'100' becomes 'ton' (ton can informally mean a hundred).
'pro'+'ton'='PROTON'
(Other definitions for proton that I've seen before include "One charged" , "Particle in atoms" , "Positive nucleon" , "Positive thing" , "Nuclear particle with positive charge" .)