Jolly good police officer, some say (5)
I believe the answer is:
super
'jolly good' is the definition.
(I've seen this before)
'police officer some say' is the wordplay.
'police officer' becomes 'supe' (I can't justify this - if you can you should believe this answer much more).
'some' indicates the central letters.
'say' becomes 'express' (to express something is to say it).
The middle of 'express' is 'r'.
'supe'+'r'='SUPER'
Can you help me to learn more?
(Other definitions for super that I've seen before include "senior cop" , "Informally great, really up there" , "Gushily great, excellent" , "Incomparable" , "Informally terrific, splendid" .)