Observe memo (4)
I believe the answer is:
note
I believe this is a double definition.
'observe' is the first definition.
(I know that observe can be written as note)
'memo' is the second definition.
(I've seen this before)
(Other definitions for note that I've seen before include "Do, re or mi?" , "A piece of paper money" , "Short written message" , "Some money - some music" , "Observe in Eton" .)