Force prisoners to exercise (9)

I believe the answer is:

constrain

'force' is the definition.
(thesaurus)

'prisoners to exercise' is the wordplay.
'prisoners' becomes 'cons'.
'to exercise' becomes 'train' (training is a kind of exercising).
'cons'+'train'='CONSTRAIN'

(Other definitions for constrain that I've seen before include "Restrict scope of (an activity)" , "Lock up" , "Severely restrict" , "Confine, limit" , "Restrict the extent of" .)

I've seen this clue in The Sun and The Telegraph.

Want a hint initially instead of a full solution? Install my app