Why is that old toilet in Habitat? (3,4)
I believe the answer is:
how come
'why' is the definition.
(synonyms)
'that old toilet in habitat?' is the wordplay.
'that old' becomes 'o' (abbreviation).
'toilet' becomes 'wc' (W.C. is an example).
'in' is an insertion indicator.
'habitat?' becomes 'home' (I've seen this in another clue).
'o'+'wc'='owc'
'owc' inserted within 'home' is 'HOW COME?'.
'is' acts as a link.
(Other definitions for how come that I've seen before include "What's the reason" , "Explain why" .)