Terrible hum -- so what recommendation from the dentist? (9)
I believe the answer is:
mouthwash
'recommendation from the dentist?' is the definition.
'terrible hum so what' is the wordplay.
'terrible' indicates anagramming the letters.
'hum'+'so'+'what'='humsowhat'
'humsowhat' anagrammed gives 'MOUTHWASH'.
(Other definitions for mouthwash that I've seen before include "Medicated solution used for gargling" , "Gargle" , "Oral rinse" , "Liquid breath freshener" , "rubbish" .)