Force prisoners to take exercise (9)
I believe the answer is:
constrain
'force' is the definition.
(thesaurus)
'prisoners to take exercise' is the wordplay.
'prisoners' becomes 'cons'.
'to take' says to put letters next to each other.
'exercise' becomes 'train' (training is a kind of exercising**).
'cons'+'train'='CONSTRAIN'
(Other definitions for constrain that I've seen before include "Hold back - non-racist (anag)" , "Confine, limit" , "Severely restrict" , "Check" , "Restrict scope of (an activity)" .)