Force prisoners on coach (9)
I believe the answer is:
constrain
'force' is the definition.
(thesaurus)
'prisoners on coach' is the wordplay.
'prisoners' becomes 'cons'.
'on' means one lot of letters go next to another.
'coach' becomes 'train' ('train' can be a synonym of 'coach').
'cons'+'train'='CONSTRAIN'
(Other definitions for constrain that I've seen before include "Check" , "Confine, limit" , "Restrict scope of (an activity)" , "Lock up" , "Force" .)