Letters sent up first class complain about motorway (3,4)
I believe the answer is:
air mail
'letters sent up' is the definition.
I can't judge whether this defines the answer.
'first class complain about motorway' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'complain' becomes 'rail' (railing is a kind of complaining).
'about' means one lot of letters goes inside another.
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'rail' going around 'm' is 'rmail'.
'ai'+'rmail'='AIR-MAIL'
Can you help me to learn more?
(Other definitions for air mail that I've seen before include "Flying post?" , "Letters taken by plane" , "Post sent overseas" , "Letter delivery system" , "Letters that are flown in" .)