Interim ruler (6)
I believe the answer is:
regent
'interim ruler' is the definition.
(a regent rules on behalf of a monarch)
This is the entire clue.
(Other definitions for regent that I've seen before include "Ruler appointed when monarch is too young or away" , "Person appointed as acting monarch" , "Ruler standing in for monarch" , "One acting for monarch who is a minor" , "acting chief" .)