Answer long letter (5)
I believe the answer is:
aitch
'letter' is the definition.
(aitch is the name for the letter H)
'answer long' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'long' becomes 'itch' (to long or itch for something is to desire it).
'a'+'itch'='AITCH'
(Other definitions for aitch that I've seen before include "Aspirated letter" , "that may be dropped" , "bone" , "Eighth letter of the English alphabet" , "The letter h in full" .)