Pooh's half-forgotten no good Hum (4)
I believe the answer is:
pong
'hum' is the definition.
(I've seen this before)
'pooh's half-forgotten no good' is the wordplay.
'half forgotten' suggests halving the letters.
'no good' becomes 'ng' (abbreviation).
'pooh' cut in half is 'po'.
'po'+'ng'='PONG'
(Other definitions for pong that I've seen before include "Horrid smell (informal)" , "hum" , "Smell unpleasantly" , "In slang, a real stink" , "Nasty smell (informal)" .)