Muslim ruler sacks revolutionary over untruth ultimately (6)
I believe the answer is:
sherif
'muslim ruler' is the definition.
(I've seen this before)
'sacks revolutionary over untruth ultimately' is the wordplay.
'sacks' becomes 'fires' ('fire' can be a synonym of 'sack').
'revolutionary' says the letters should be written backwards (letters revolve around).
'over' indicates putting letters inside.
'ultimately' says to take the final letters.
The final letter of 'untruth' is 'h'.
'fires' back-to-front is 'serif'.
'serif' placed around 'h' is 'SHERIF'.
(Other definitions for sherif that I've seen before include "Arab leader" , "Muslim leader" , "Muslim chief" , "Muslim ruler" .)