Only one warning in four is heard (4)

I believe the answer is:

fore

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'only one warning' is the definition.
'fore' can be an answer for 'warning' (shouted in golf). I am unsure of the 'only one' bit.

'four is heard' is the wordplay.
'is heard' shows a homophone (sound like).
'four' sounds like 'FORE'.

'in' acts as a link.

Can you help me to learn more?

(Other definitions for fore that I've seen before include "A golfer's warning cry - ball in flight" , "Ancestor" , "Fairway warning" , "The front part of a ship" , "Golfer's warning shout" .)

I've seen this clue in the Evening Standard.

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