A high post or I'm a liar (7)
I believe the answer is:
airmail
'a high post' is the definition.
'airmail' can be an answer for 'post' (airmail is a kind of post). I am not sure about the 'a high' bit.
'i'm a liar' is the wordplay.
'i'm a liar' anagrams to 'AIRMAIL'.
But, I'm not sure how the anagram is indicated.
'or' acts as a link.
Can you help me to learn more?
(Other definitions for airmail that I've seen before include "Letters taken by plane" , "Flying post?" , "It's marked "par avion"" , "Quick way to deliver" , "Postal service" .)