Observe a memo (4)

I believe the answer is:

note

I believe this clue is a double definition.

'observe' is the first definition.
(I know that observe can be written as note)

'a memo' is the second definition.
(I've seen this before)

(Other definitions for note that I've seen before include "Cash" , "A, B or C, for example - short message" , "Memo; tone" , "Short written message" , "Jotting" .)

I've seen this clue in The Mirror.

Want a hint initially instead of a full solution? Install my app