Indefensible and inadequate, housing a number (9)
I believe the answer is:
untenable
'indefensible' is the definition.
(synonyms)
'inadequate housing a number' is the wordplay.
'inadequate' becomes 'unable' (I can't justify this - if you can you should give a lot more credence to this answer).
'housing' means one lot of letters goes inside another.
'a number' becomes 'ten' (ten is a number).
'unable' placed around 'ten' is 'UNTENABLE'.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for untenable that I've seen before include "impossible to support" , "Incapable of being defended or justified" , "Incapable of being maintained or defended" , "Not defensible" , "Indefensible" .)