Eastern potentate back in Crimea (4)
I believe the answer is:
emir
'eastern potentate' is the definition.
(I've seen this in another clue)
'back in crimea' is the wordplay.
'back' shows that the letters should be reversed in order.
'in' indicates a hidden word.
'rime' is hidden in the letters of 'crimea'.
'rime' back-to-front is 'EMIR'.
(Other definitions for emir that I've seen before include "Ruler or chieftain in Arabia" , "Title of various rulers; I'm ER (anag.)" , "Arabian ruler" , "Arab ruler" , "Top man" .)