Officer belonging to precinct protecting chief of police (9)
I believe the answer is:
inspector
'officer' is the definition.
(I know that inspector is a type of officer)
'belonging to precinct protecting chief of police' is the wordplay.
'belonging to' becomes 'in' (in can mean being a member of).
'precinct' becomes 'sector' (both can mean an area or zone).
'protecting' means one lot of letters goes inside another (inserted letters are held or protected).
'chief of' suggests taking the first letters.
The initial letter of 'police' is 'p'.
'sector' enclosing 'p' is 'spector'.
'in'+'spector'='INSPECTOR'
(Other definitions for inspector that I've seen before include "Examining official" , "Policeman; compliance official" , "reviewer" , "controller" , "auditor" .)