Serf left during rescue (5)
I believe the answer is:
slave
'serf' is the definition.
(I've seen this in another clue)
'left during rescue' is the wordplay.
'left' becomes 'l' (common abbreviation).
'during' indicates putting letters inside.
'rescue' becomes 'save' (saving is a kind of rescuing).
'l' put into 'save' is 'SLAVE'.
(Other definitions for slave that I've seen before include "Bondsman, thrall" , "Person who is the property of another" , "Drudge" , "Unpaid worker" , "Person in bondage" .)