Written message to recall missing rays? Odd! (4)
I believe the answer is:
memo
'written message' is the definition.
'memo' can be an answer for 'message' (memo is a kind of message). I am not certain of the 'written' bit.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for memo that I've seen before include "In brief, a note of events" , "Reminder (abbr)" , "A little something to be remembered" , "It will remind" , "- Currie, Br. politician" .)