Chart useful in semaphore (3)
I believe the answer is:
map
'chart useful in semaphore' is the definition.
'map' can be an answer for 'chart' ('map' can be a synonym of 'chart'). I'm unsure of the 'useful in semaphore' bit.
'in semaphore' is the wordplay.
'in' says the answer is hidden in the clue.
'MAP' is hidden in the letters of '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" .)