Copy a letter out, being backward (3)

I believe the answer is:

coy

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'being backward' is the definition.
(thesaurus)

'copy a letter' is the wordplay.
'copy' becomes 'co' (I can't explain this - if you can you should believe this answer much more).
'a letter' becomes 'y' (Y is a letter of the alphabet).
'co'+'y'='COY'

'out' acts as a link.

Can you help me to learn more?

(Other definitions for coy that I've seen before include "Reserved" , "not forthcoming" , "Pretending modesty" , "Pretending shyness" , "Demure, like Marvell's mistress" .)

I've seen this clue in the Evening Standard.

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