Threatens fellows with experts (7)
I believe the answer is:
menaces
'threatens' is the definition.
('menace' can be a synonym of 'threaten')
'fellows with experts' is the wordplay.
'fellows' becomes 'men' (fellow can mean a man).
'with' means one lot of letters go next to another.
'experts' becomes 'aces' (ace is a kind of expert).
'men'+'aces'='MENACES'
(Other definitions for menaces that I've seen before include "Threats" , "Threatens" .)