An urge for a letter (5)
I believe the answer is:
aitch
'letter' is the definition.
(aitch is a kind of letter)
'an urge for a' is the wordplay.
'an urge' becomes 'itch' (I've seen this before).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'itch' put after 'a' is 'AITCH'.
(Other definitions for aitch that I've seen before include "Letter sound that's often dropped?" , "Eighth letter of the English alphabet" , "that I will immediately succeed" , "character leading heroes" , "first bit of hymn?" .)