Taking off, aircraft's beginning to make high-pitched sound (5)
I believe the answer is:
aping
'taking off' is the definition.
(I've seen this before)
'aircraft's beginning to make high-pitched sound' is the wordplay.
'beginning' suggests taking the first letters.
'to make high-pitched sound' becomes 'ping'.
The initial letter of 'aircrafts' is 'a'.
'a'+'ping'='APING'
(Other definitions for aping that I've seen before include "Imitation" , "Taking off" , "Sending up" , "Making copy of" , "Parroting" .)