Hooter sound one's missing (4)
I believe the answer is:
nose
'hooter' is the definition.
(informal term)
'sound one's missing' is the wordplay.
'sound' becomes 'noise' (a noise is a sound).
'one' becomes 'i' (Roman numeral).
'missing' indicates named letters should be taken away.
'noise' with 'i' taken away is 'NOSE'.
(Other definitions for nose that I've seen before include "Conk (slang)" , "Don't cut this off to spite your face" , "In-your-face" , "Schnozzle" , "Facial feature" .)