Detective heading tough precinct (8)
I believe the answer is:
district
'precinct' is the definition.
(thesaurus)
'detective heading tough' is the wordplay.
'detective' becomes 'di' (Detective Inspector abbreviation).
'heading' is a charade indicator (letters next to each other) (some letters go in front of or head others).
'tough' becomes 'strict' (I've seen this before).
'di'+'strict'='DISTRICT'
(Other definitions for district that I've seen before include "Precinct, section" , "'Area, territory (8)'" , "Region marked off for administrative or other purposes" , "Zone, area" , "Area (of a town/city)" .)