Having no hooter keeps one quiet (9)
I believe the answer is:
noiseless
'one quiet' is the definition.
(I've seen this in another clue)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for noiseless that I've seen before include "Lionesses (anag) - making no sound" , "unsound?" , "Silent, quiet" , "with a silencer?" , "Mum" .)