Chart useful in semaphore (3)

I believe the answer is:

map

I'm a little stuck... Click here to teach me more about this clue!

'chart useful in semaphore' is the definition.
'map' can be an answer for 'chart' (synonyms). I am not sure about the 'useful in semaphore' bit.

'in semaphore' is the wordplay.
'in' indicates the answer is hidden within the clue.
'MAP' is hidden within 'semaphore'.

Can you help me to learn more?

(Other definitions for map that I've seen before include "Diagram of an area" , "Travel guide" , "Chart" , "Geographical plan" , "(Make a) geographical representation" .)

I've seen this clue in the Evening Standard.

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