A long envelope for express letters (7)
I believe the answer is:
aitches
'express letters' is the definition.
'aitches' can be an answer for 'letters' (I've seen this before). I am not sure about the 'express' bit.
'a long envelope' is the wordplay.
'long' becomes 'itch' (to long or itch for something is to desire it).
'envelope' becomes 'es' (I can't explain this - if you can you should believe this answer much more).
'a'+'itch'+'es'='AITCHES'
'for' is the link.
Can you help me to learn more?
(Other definitions for aitches that I've seen before include "They are regularly dropped in the East End" , "Two of which are in Hythe" , "Letters" .)