After a time the doctor writes back a letter (5)

I believe the answer is:

omega

I'm a little stuck... Click here to teach me more about this clue!

'letter' is the definition.
(I know that omega is a type of letter)

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

Can you help me to learn more?

(Other definitions for omega that I've seen before include "Conclusion" , "Ending of a series or sequence" , "symbol of resistance?" , "long letter" , "The last in a series of letters" .)

I've seen this clue in the Evening Standard.

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