Revolutionary leader in Marseilles with a deserter (5)
I believe the answer is:
marat
'revolutionary' is the definition.
(French revolutionary Jean-Paul Marat)
'leader in marseilles with a deserter' is the wordplay.
'leader in' says to take the initial letters.
'with' says to put letters next to each other.
'deserter' becomes 'rat' (both can mean a traitor).
The initial letter of 'marseilles' is 'm'.
'm'+'a'+'rat'='MARAT'
(Other definitions for marat that I've seen before include "revolutionary stabbed" , "French revolutionary killed by Charlotte Corday" , "French revolutionary killed in his bath" , "Charlotte Corday murdered him in his bath" , "Jean Paul -, assassinated French revolutionary" .)