Exactly right function, mainly - for a police officer? (6)
I believe the answer is:
patrol
'a police officer?' is the definition.
(term for a police officer patrolling an area)
'exactly right function mainly' is the wordplay.
'exactly right' becomes 'pat' (pat can mean perfect or over-rehearsed).
'function' becomes 'role' (synonyms).
'mainly' means to remove the last letter.
'role' with its last letter taken away is 'rol'.
'pat'+'rol'='PATROL'
'for' acts as a link.
(Other definitions for patrol that I've seen before include "Regular beat of a guard" , "Move round and keep watch" , "Scouts or Guides group" , "Portal for those on security duty" , "Go on one's rounds" .)