Message in bottle oddly missing (4)
I believe the answer is:
note
'message' is the definition.
(note is a kind of message)
'in bottle oddly missing' is the wordplay.
'oddly missing' means one should take alternating letters.
'in'+'bottle'='inbottle'
The alternating letters of 'inbottle' are 'NOTE'.
(Other definitions for note that I've seen before include "Record; memo" , "Tone of distinction" , "Tone of importance" , "Observe in Eton" , "Write down" .)