Short informal letter (4)

I think it could be either:

note

loos

'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" .)

I've seen this clue in The Telegraph.

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