After a time the doctor writes back a letter (5)
I believe the answer is:
omega
'letter' is the definition.
(omega is a kind of letter)
I don't understand how the rest 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" .)