Expert encounters weight in particle (6)
I believe the answer is:
proton
'particle' is the definition.
(proton is a kind of particle)
'expert encounters weight' is the wordplay.
'expert' becomes 'pro' (I've seen this before).
'encounters' is a charade indicator (letters next to each other).
'weight' becomes 'ton' (ton is a kind of weight).
'pro'+'ton'='PROTON'
'in' is the link.
(Other definitions for proton that I've seen before include "Particle in atom nucleus" , "One's charged" , "Subatomic particle with positive charge" , "Positively-charged particle" , "Elementary particle found in all atomic nuclei" .)