Expert needs weight for elementary particle (6)
I believe the answer is:
proton
'elementary particle' is the definition.
(I've seen this before)
'expert needs weight' is the wordplay.
'expert' becomes 'pro' (I've seen this before).
'needs' means one lot of letters go next to another.
'weight' becomes 'ton' (ton is a kind of weight).
'pro'+'ton'='PROTON'
'for' is the link.
(Other definitions for proton that I've seen before include "Stable subatomic particle" , "Subatomic particle with positive charge" , "One charged" , "just a little bit" , "Positively charged particle" .)