Residence has zero facilities inside - why? (3,4)
I believe the answer is:
how come
'why?' is the definition.
('how come' can be a synonym of 'why')
'residence has zero facilities' is the wordplay.
'residence' becomes 'home' (home is a kind of residence).
'has' means one lot of letters goes inside another.
'zero' becomes 'O' (O resembles 0).
'facilities' becomes 'wc' (water closet).
'o'+'wc'='owc'
'home' going around 'owc' is 'HOW COME?'.
'inside' acts as a link.
(Other definitions for how come that I've seen before include "What's the reason" , "Explain why" .)