A heartless measure for Islamic ruler (5)
I believe the answer is:
ameer
'islamic ruler' is the definition.
(I've seen this in another clue)
'a heartless measure' is the wordplay.
'heartless measure' becomes 'meer' (I can't justify this - if you can you should believe this answer much more).
'a'+'meer'='AMEER'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for ameer that I've seen before include "Asian dignitary" , "Muslim leader" , "Eastern potentate (variant spelling)" , "foreign ruler" , "Former ruler's title in Afghanistan" .)