But not too backward to copy a letter out! (3)
I believe the answer is:
coy
'but not too backward' is the definition.
'coy' can be an answer for 'backward' (thesaurus). I'm not sure about the 'but not too' bit.
'copy a letter out?' is the wordplay.
I cannot really understand how this works, but
'letter' could be 'y' (Y is a letter of the alphabet) and 'y' is found within the answer.
The remaining letters 'co' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for coy that I've seen before include "Military team in brief" , "Reserved" , "Demure, like Marvell's mistress" , "Pretending shyness" , "Pretending modesty" .)