Is familiar with the sound of a hooter (4)
I believe the answer is:
nose
'a hooter' is the definition.
(informal term)
'is familiar with the sound' is the wordplay.
'is familiar with' becomes 'knows' (to know something is to be familiar with it).
'the sound' shows a homophone (sound like).
'knows' sounds like 'NOSE'.
'of' acts as a link.
(Other definitions for nose that I've seen before include "Organ of face" , "Sense of smell; proceed cautiously" , "Hooter" , "Schnozzle" , "Pry" .)