Philip beginning to examine tweet (4)
I believe the answer is:
pipe
'tweet' is the definition.
(both can mean to make a high-pitched sound)
'philip beginning to examine' is the wordplay.
'philip' becomes 'pip' (can be a nickname for Philip).
'beginning to' suggests taking the first letters.
The initial letter of 'examine' is 'e'.
'pip'+'e'='PIPE'
(Other definitions for pipe that I've seen before include "Hollow tube or musical instrument" , "Something in the organ loft" , "Utter shrilly; wine cask" , "Main provider" , "Part of an organ; tube" .)