What needs scratching on a letter (5)

I believe the answer is:

aitch

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'letter' is the definition.
(aitch is the name for the letter H)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for aitch that I've seen before include "bone" , "It's dropped by Cockneys" , "Aspirated letter" , "what some drop?" , "The letter h in full" .)

I've seen this clue in The Independent.

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