Army takes mature prisoner (7)
I believe the answer is:
hostage
'prisoner' is the definition.
(hostage is a kind of prisoner)
'army takes mature' is the wordplay.
'army' becomes 'host' (I've seen this before).
'takes' means one lot of letters go next to another.
'mature' becomes 'age' ('age' can be a synonym of 'mature').
'host'+'age'='HOSTAGE'
(Other definitions for hostage that I've seen before include "One held for ransom" , "One held for money" , "She-goat (anag)" , "Person held by kidnapper for ransom" , "Captive held for ransom" .)