Only high fliers may take this post (7)
I believe the answer is:
airmail
'only high fliers may take this post' is the definition.
'airmail' can be an answer for 'post' (airmail is a kind of post). I'm unsure of the remainder of the definition.
'fliers may take this post' is the wordplay.
'fliers may' becomes 'air' (I can't justify this - if you can you should believe this answer much more).
'take' means one lot of letters go next to another.
'this post' becomes 'mail' (synonyms).
'air'+'mail'='AIRMAIL'
Can you help me to learn more?
(Other definitions for airmail that I've seen before include "Postal service" , "Quick way to deliver" , "Letters taken by plane" , "I'm a liar (anag)" , "Letters that are flown in" .)