Force prisoners to drill? (9)
I believe the answer is:
constrain
'force' is the definition.
(I've seen this before)
'prisoners to drill?' is the wordplay.
'prisoners' becomes 'cons' (short for convicts).
'to drill?' becomes 'train' (drilling is a kind of training).
'cons'+'train'='CONSTRAIN'
(Other definitions for constrain that I've seen before include "Severely restrict" , "Force" , "Compel - limit" , "Compel; constrict" , "Hold back - non-racist (anag)" .)