Threat posed by fellow expert (6)
I believe the answer is:
menace
'threat' is the definition.
('menace' can be a synonym of 'threat')
'fellow expert' is the wordplay.
'fellow' becomes 'men' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'expert' becomes 'ace' (ace is a kind of expert).
'men'+'ace'='MENACE'
'posed by' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Other definitions for menace that I've seen before include "peril" , "serious nuisance" , "Intimidate" , "Threat, danger" , "Threat; threaten" .)