Some grim Easterner, revolutionary ruler (4)
I believe the answer is:
emir
'ruler' is the definition.
(emir is a kind of ruler)
'some grim easterner revolutionary' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'revolutionary' shows that the letters should be reversed in order.
'rime' is hidden within 'grim easterner'.
'rime' reversed gives 'EMIR'.
(Other definitions for emir that I've seen before include "Islamic title" , "Eastern ruler in mire" , "Islamic chieftain" , "Islamic ruler" , "Arab prince" .)