Specialist continued to handle Muslim ruler (10)
I believe the answer is:
consultant
'specialist' is the definition.
(medical specialist)
'continued to handle muslim ruler' is the wordplay.
'continued' becomes 'cont' (common abbreviation).
'to handle' means one lot of letters goes inside another (handle can mean receive or take possession of).
'muslim ruler' becomes 'sultan' (title for a royal in some Muslim countries).
'cont' going around 'sultan' is 'CONSULTANT'.
(Other definitions for consultant that I've seen before include "Adviser" , "See 1 across" , "Senior hospital doctor" , "Specialist" .)