Police officer more prominent in series . . . (5)
I believe the answer is:
super
'police officer' is the definition.
(term for a senior police officer)
'more prominent in series' is the wordplay.
'more prominent' becomes 'up' (I can't explain this - if you can you should believe this answer much more).
'in' indicates putting letters inside.
'series' becomes 'ser' (abbreviation for series).
'up' put inside 'ser' is 'SUPER'.
Can you help me to learn more?
(Other definitions for super that I've seen before include "Wicked" , "Brilliant" , "Delightful" , "Incomparable" , "Jolly good" .)