Force prisoners on transport (9)
I believe the answer is:
constrain
'force' is the definition.
(thesaurus)
'prisoners on transport' is the wordplay.
'prisoners' becomes 'cons'.
'on' is a charade indicator (letters next to each other).
'transport' becomes 'train' (train is a kind of transport**).
'cons'+'train'='CONSTRAIN'
(Other definitions for constrain that I've seen before include "Compel; constrict" , "Restrict the extent of" , "Lock up" , "Check" , "Compel towards a course of action" .)