Do in (2-4)
(Other definitions for at-home that I've seen before include "Available to visitors, quite comfortable" , "In (for a party?)" , "Not away, ready for visitors" , "Comfortable - in one's own country" , "Former Italian currency" .)
'in?' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'do' is the wordplay.
I cannot quite see how this works, but
'do' could be 'party' (I've seen this before) and 'party' is found within the answer.
The remaining letters 'house' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for house party that I've seen is " Group of guests staying a few days".)