The guy left, to return as a slave (5)
I believe the answer is:
helot
'a slave' is the definition.
(slave in ancient Sparta)
'the guy left to return' is the wordplay.
'the guy' becomes 'he'.
'left' becomes 'l' (common abbreviation).
'return' shows that the letters should be reversed in order.
'to' written backwards gives 'ot'.
'he'+'l'+'ot'='HELOT'
'as' is the link.
(Other definitions for helot that I've seen before include "downtrodden?" , "Hotel (anag) - serf" , "Greek serf" , "Serf of ancient Sparta" , "One made to work" .)