Rogue ultimately splits faction (5)
I believe the answer is:
scamp
'rogue' is the definition.
(I've seen this before)
'ultimately splits faction' is the wordplay.
'ultimately' suggests the final letters.
'faction' becomes 'camp' (both can mean a side in an argument).
The last letter of 'splits' is 's'.
's'+'camp'='SCAMP'
(Other definitions for scamp that I've seen before include "Scoundrel" , "Impish child" , "pickle" , "Impish youngster" , "Scallywag" .)