Short informal letter (4)
'short informal letter' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for note that I've seen before include "Tone of a short record" , "Heed; short letter" , "Observation" , "Sound, record" , "Comment" .)
(Other definitions for loos that I've seen before include "Conveniences" , "Austrian architect" .)