Said the letter travels by air (3)
I believe the answer is:
bee
'said the letter travels by air' is the definition.
I can't tell whether this definition defines the answer.
'said the letter' is the wordplay.
'said' indicates a 'sounds like' (homophone) clue.
'the letter' becomes 'b'.
'b' is a homophone of 'BEE'.
Can you help me to learn more?
(Other definitions for bee that I've seen before include "Busy buzzer" , "Hive worker" , "Insect that bumbles?" , "Hymenopterous insect" , "Honey generator" .)