Great trouble concealing jollies in high-flying post (7)
I believe the answer is:
airmail
'high-flying post' is the definition.
'airmail' can be an answer for 'post' (airmail is a kind of post). I am not sure about the 'high-flying' bit.
'great trouble concealing jollies' is the wordplay.
'great' becomes 'AI' (resembles A1, old-fashioned term meaning very good).
'trouble' becomes 'ail' (synonyms).
'concealing' indicates putting letters inside.
'jollies' becomes 'RM' (nickname for the Royal Marines).
'ai'+'ail'='aiail'
'aiail' placed around 'rm' is 'AIRMAIL'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for airmail that I've seen before include "Postal service" , "Letters that are flown in" , "It's marked "par avion"" , "overseas post?" , "Flying post?" .)