Jack suffered behind bars (6)
I believe the answer is:
jailed
'behind bars' is the definition.
(I've seen this in another clue)
'jack suffered' is the wordplay.
'jack' becomes 'J' (playing card abbreviation).
'suffered' becomes 'ailed' (ailing is a kind of suffering).
'j'+'ailed'='JAILED'
(Other definitions for jailed that I've seen before include "in jug" , "Incarcerated" , "Put away" .)