Governor in favour of prisoner uniform in small and large (9)
I believe the answer is:
proconsul
'governor' is the definition.
(I've seen this before)
'in favour of prisoner uniform in small and large' is the wordplay.
'in favour of' becomes 'pro' (I've seen this before).
'prisoner' becomes 'con' (abbreviation for 'convict').
'uniform in small' becomes 'su' (I am not sure about this - if you are sure you should believe this answer much more).
'and' means one lot of letters go next to another.
'large' becomes 'l'.
'pro'+'con'+'su'+'l'='PROCONSUL'
(Other definitions for proconsul that I've seen before include "magistrate" , "Ancient Roman provincial governor" .)