Obsolete furnace regularly comes in handy (7)
I believe the answer is:
defunct
'obsolete' is the definition.
(thesaurus)
'furnace regularly comes in handy' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'comes in' is an insertion indicator.
'handy' becomes 'deft' (I've seen this before).
The alternating letters of 'furnace' are 'unc'.
'unc' put inside 'deft' is 'DEFUNCT'.
(Other definitions for defunct that I've seen before include "dead and gone" , "Obsolete" , "'No longer in force, inactive (7)'" , "no longer valid" , "No longer operative" .)