Many grew old behind bars (5)
I believe the answer is:
caged
'behind bars' is the definition.
(I've seen this before)
'many grew old' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for 100).
'grew old' becomes 'aged' ('age' can be a synonym of 'grow old').
'c'+'aged'='CAGED'
(Other definitions for caged that I've seen before include "Cooped up" , "Confined behind bars" , "Shut up" , "In a zoo?" , "in prison" .)