Pro I forewarn’s to become a captive (8,2,3)
I believe the answer is:
prisoner of war
'a captive' is the definition.
(prisoner of war is a kind of captive)
'pro i forewarn's to become' is the wordplay.
'to become' indicates an anagram.
'pro'+'i'+'forewarns'='proiforewarns'
'proiforewarns' is an anagram of 'PRISONER OF WAR'.
(Other definitions for prisoner of war that I've seen before include "Captured soldier" , "Captured member of enemy forces" , "Enemy captive" , "E.g. Stalag inmate" , "POW in full" .)