Bill's old schoolmaster (4)
I believe the answer is:
beak
This clue could be a double definition.
'bill's' is the first definition.
(synonyms)
'old schoolmaster' is the second definition.
'beak' can be an answer for 'schoolmaster' (I've seen this before). I am not sure about the 'old' bit.
Can you help me to learn more?
(Other definitions for beak that I've seen before include "Schoolmarm" , "Horny mouth of a bird" , "Judge - bill" , "Mandible" , "Bird's bill" .)