Hold back prisoner with small drill (9)
I believe the answer is:
constrain
'hold back' is the definition.
(I've seen this in another clue)
'prisoner with small drill' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'with' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'drill' becomes 'train' (I've seen this in another clue).
'con'+'s'+'train'='CONSTRAIN'
(Other definitions for constrain that I've seen before include "Hold back - non-racist (anag)" , "Confine, limit" , "Force" , "Compel towards a course of action" , "Restrict scope of (an activity)" .)