Army gets on with prisoners (8)
I believe the answer is:
hostages
'prisoners' is the definition.
(hostage is a kind of prisoner)
'army gets on' is the wordplay.
'army' becomes 'host' (I've seen this before).
'gets on' becomes 'ages' ('age' can be a synonym of 'get on').
'host'+'ages'='HOSTAGES'
'with' is the link.
(Other definitions for hostages that I've seen before include "Victims of kidnap" , "People detained" , "They're held as sureties against demands" , "Prisoners held as sureties" .)