The measure of a leader? (5)
I believe the answer is:
ruler
I believe this clue is a double definition.
'the measure' is the first definition.
(ruler is a kind of measure)
'leader?' is the second definition.
(I know that ruler is a type of leader)
'of a' acts as a link.
(Other definitions for ruler that I've seen before include "One in power" , "Eg, king, emperor" , "Governor or drawing stick" , "King, president etc." , "Monarch or measure" .)