Across the water, is only half a mile! (4)

I believe the answer is:

isle

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'across the water' is the definition.
I can't tell whether this definition defines the answer.

'is only half a mile' is the wordplay.
'only half' means to take half the letters.
'mile' cut in half is 'le'.
'is'+'le'='ISLE'

Can you help me to learn more?

(Other definitions for isle that I've seen before include "This is sceptred!" , "Atoll" , "It lies badly in water" , "eg Hoy, Yell or Sark" , "Man's one" .)

I've seen this clue in the Evening Standard.

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