Jack was sick in prison (6)
I believe the answer is:
jailed
'in prison' is the definition.
(thesaurus)
'jack was sick' is the wordplay.
'jack' becomes 'J' (playing card abbreviation).
'was sick' becomes 'ailed' (I've seen this before).
'j'+'ailed'='JAILED'
(Other definitions for jailed that I've seen before include "Put away" , "Incarcerated" , "in jug" .)