Arab ruler retaining good worker, one about to leave? (8)
I believe the answer is:
emigrant
'one about to leave?' is the definition.
(someone leaving a country)
'arab ruler retaining good worker' is the wordplay.
'arab ruler' becomes 'emir' (type of ruler in some countries).
'retaining' means one lot of letters goes inside another (inserted letters are held or retained).
'good' becomes 'g' (abbreviation).
'worker' becomes 'ant' (worker is a type of ant).
'emir' enclosing 'g' is 'emigr'.
'emigr'+'ant'='EMIGRANT'
(Other definitions for emigrant that I've seen before include "Time Gran went abroad to settle" , "One leaving for another country" , "One who leaves to live in another country" , "Write carelessly" , "one on the way out?" .)