Mum, on air, drunk on unknown wine (6)
I believe the answer is:
shiraz
'wine' is the definition.
(I've seen this before)
'mum on air drunk on unknown' is the wordplay.
'mum' becomes 'sh' (I've seen this in other clues).
'on air drunk' becomes 'ira' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' says to put letters next to each other.
'unknown' becomes 'z' (algebra).
'sh'+'ira'+'z'='SHIRAZ'
(Other definitions for shiraz that I've seen before include "18 [WINE]" , "Variety of black grape" , "Iran city; Aussie grape" , "Variety of red wine" , "Iranian city - variety of wine grape" .)