It's rude to have house-warming without one (9)
I believe the answer is:
Here is my best explanation:
'it's rude' is the definition.
'house-warming without one' is the subsidiary indication.
'house-warming' becomes 'insulating'.
'without' indicates named letters should be taken away.
'one' becomes 'a'.
'insulating' with 'a' removed is 'insulting'.
'to have' is the link.
(Other definitions for insulting that I've seen before include "Treating with disrespect", "What offender's doing" and "Offensive".)