Muslim leader in semi-retirement (4)
I believe the answer is:
emir
'muslim leader' is the definition.
(monarchic title in some Muslim countries)
'in semi-retirement' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EMIR' can be found hidden inside 'semi retirement'.
(Other definitions for emir that I've seen before include "Local chief" , "Title of various Muslim rulers" , "Arab prince" , "Islamic title" , "Middle-Eastern ruler" .)