The ability to make a prisoner royal (5)
I believe the answer is:
power
'the ability to make' is the definition.
(I've seen this before)
'a prisoner royal' is the wordplay.
'a prisoner' becomes 'pow' (POW is an example).
'royal' becomes 'er' (I've seen this in other clues).
'pow'+'er'='POWER'
(Other definitions for power that I've seen before include "See 12" , "index" , "Strength - capacity" , "Control and influence exercised over others" , "Physical force, ability" .)