A long letter (5)
I believe the answer is:
aitch
'letter' is the definition.
(aitch is a kind of letter)
'a long' is the wordplay.
'long' becomes 'itch' (I've seen this before).
'a'+'itch'='AITCH'
(Other definitions for aitch that I've seen before include "first bit of hymn?" , "what some drop?" , "bone" , "The letter h in full" , "It's dropped by Cockneys" .)