Housing unit, number one of two (6)
I believe the answer is:
either
'one of two' is the definition.
(I've seen this before)
'housing unit number' is the wordplay.
'housing' is an insertion indicator.
'unit' becomes 'i' (synonyms).
'number' becomes 'ether' (anaesthetic, something which numbs).
'i' placed into 'ether' is 'EITHER'.
(Other definitions for either that I've seen before include "Her tie (anag.)" , "offered a choice" , "could be one of two" , "Each of two; moreover" , "Word introducing first of two 16s" .)