Ruler from European border heading West (4)
I believe the answer is:
emir
'ruler' is the definition.
(historical term for a Muslim ruler)
'european border heading west' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'border' becomes 'rim' (I've seen this in another clue).
'heading west' shows that the letters should be reversed in order.
'rim' back-to-front is 'mir'.
'e'+'mir'='EMIR'
'from' acts as a link.
(Other definitions for emir that I've seen before include "leader in the East" , "Middle Eastern ruler" , "Title of various Muslim rulers" , "eader abroad?" , "Middle-Eastern ruler" .)