Immense downside of love letter (5)
I believe the answer is:
omega
'letter' is the definition.
(I know that omega is a type of letter)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for omega that I've seen before include "Ending of a series or sequence" , "Traditional story" , "character from Athens" , "Ultimate letter" , "24th Greek letter" .)