Arab ruler in semi-retirement (4)
I believe the answer is:
emir
'arab ruler' is the definition.
(I've seen this before)
'in semi-retirement' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EMIR' is hidden within 'semi retirement'.
(Other definitions for emir that I've seen before include "Islamic title" , "leader in the East" , "Arab ruler" , "Islamic ruler" , "Title of various rulers; I'm ER (anag.)" .)