Detective attending houses in part of mine (4)
I believe the answer is:
adit
'part of mine' is the definition.
('adit' is part of 'mine')
'detective attending houses' is the wordplay.
'detective' becomes 'di' (Detective Inspector abbreviation).
'attending' becomes 'at' (e.g. at school means attending school).
'houses' indicates putting letters inside.
'di' placed into 'at' is 'ADIT'.
'in' acts as a link.
(Other definitions for adit that I've seen before include "Mine entrance passage" , "Da, it is a mine entrance" , "Mine-shaft" , "Horiz. mine entrance" , "Means of access to mine" .)