Post in flying? (7)
I believe the answer is:
airmail
'post in flying?' is the definition.
'airmail' can be an answer for 'post' (airmail is a kind of post). I'm not certain of the 'in flying?' bit.
'post in flying?' is the wordplay.
I cannot really see how this works, but
'post' could be 'mail' (synonyms) and 'mail' is found within the answer.
'flying?' could be 'air' ('air' can be similar in meaning to 'flying') and 'air' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for airmail that I've seen before include "overseas post?" , "Postal system f" , "Letters carried by plane" , "Letter delivery system" , "Postal service" .)