Particle identified by Greek character, an Athenian demagogue (7)
I believe the answer is:
nucleon
'particle' is the definition.
(nucleon is a kind of particle)
'greek character an athenian demagogue' is the wordplay.
'greek character' becomes 'nu' (letter from the Greek alphabet).
'an athenian demagogue' becomes 'cleon' (ancient Greek general).
'nu'+'cleon'='NUCLEON'
'identified by' is the link.
(Other definitions for nucleon that I've seen before include "Particle" , "fundamental constituent" .)