Mitford sister hosts celebrated designer Mary X perhaps (7,8)
I believe the answer is:
unknown quantity
'mitford sister hosts celebrated designer' is the definition.
I can't tell whether this defines the answer.
'mary x perhaps' is the wordplay.
'mary' becomes 'unknown' (I can't explain this - if you can you should give a lot more credence to this answer).
'x perhaps' becomes 'quantity' (x is a kind of quantity).
'unknown'+'quantity'='UNKNOWN QUANTITY'
Can you help me to learn more?
(Other definitions for unknown quantity that I've seen before include "Candidate not yet examined" , "any one of the last three characters?" , "Dark horse" , "one of those following VW" , "may be last in Derby, literally" .)