Means of sending by air (5)
I believe the answer is:
radio
This might be a double definition.
'means of sending' is the first definition.
'radio' can be an answer for 'means' (I have seen 'means of communication' mean 'radio' so perhaps 'means' could also mean 'radio'). I am unsure of the 'of sending' bit.
'air' is the second definition.
(thesaurus)
'by' is the link.
Can you help me to learn more?
(Other definitions for radio that I've seen before include "Communications device" , "Part of the broadcasting industry" , "Electronic medium of communication" , "Marconi invented it" , "Wireless broadcast" .)