It is bird island (7)

I believe the answer is:

martini

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'it' is the definition.
The definition suggests a singular noun which matches the answer.

'bird island' is the wordplay.
'bird' becomes 'martin' (martin is a kind of bird).
'island' becomes 'i' (geographical abbreviation).
'martin'+'i'='MARTINI'

'is' acts as a link.

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(Other definitions for martini that I've seen before include "Type of Italian vermouth" , "rifle" , "Sienese painter" , "Italian drink" , "James Bond's drink of choice" .)

I've seen this clue in The Telegraph.

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