Force prisoners on to coach (9)
I believe the answer is:
constrain
'force' is the definition.
(I've seen this before)
'prisoners on to coach' is the wordplay.
'prisoners' becomes 'cons' (short for convicts).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'to coach' becomes 'train' (both can mean to instruct a sportsperson).
'cons'+'train'='CONSTRAIN'
(Other definitions for constrain that I've seen before include "Confine, limit" , "Severely restrict" , "Lock up" , "Hold back - non-racist (anag)" , "Force" .)