The man's mostly unwilling to be a slave (5)
I believe the answer is:
helot
'a slave' is the definition.
(I've seen this before)
'the man's mostly unwilling' is the wordplay.
'the man' becomes 'he' (**).
'mostly' means to remove the last letter.
'unwilling' becomes 'loth' ('loth' can be similar in meaning to 'unwilling').
'loth' with its last letter taken off is 'lot'.
'he'+'lot'='HELOT'
'to be' acts as a link.
(Other definitions for helot that I've seen before include "Spartan slave" , "Spartan serf" , "One made to work" , "downtrodden?" , "Serf of ancient Sparta" .)