Fictional detective left in houses (6)
I believe the answer is:
holmes
'fictional detective' is the definition.
(Sherlock Holmes)
'left in houses' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'houses' becomes 'homes' (homing is a kind of housing).
'l' put within 'homes' is 'HOLMES'.
(Other definitions for holmes that I've seen before include "See 3" , "Baker Street detective" , "Conan Doyle creation" , "Conan Doyle's 18" , "Famous detective" .)