Improve control over prisoner, substantial troublemaker (11)
I believe the answer is:
consolidate
'improve control' is the definition.
(as in consolidating one's power)
'over prisoner substantial troublemaker' is the wordplay.
'over' means one lot of letters go next to another.
'prisoner' becomes 'con' (abbreviation for 'convict').
'substantial' becomes 'solid' (synonyms).
'troublemaker' becomes 'ate' (I've seen this in other clues).
'con' put next to 'solid' is 'consolid'.
'consolid'+'ate'='CONSOLIDATE'
(Other definitions for consolidate that I've seen before include "Combine into a connected whole" , "Make stronger; combine into one" , "Join together - strengthen" , "Make firm" , "Fuse" .)