Basis for measuring this? "Gut feel: London” I twice elaborated (4,2,9)

I believe the answer is:

line of longitude

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'basis for measuring this?' is the definition.
The definition suggests a singular noun which matches the answer.

'gut feel london i twice elaborated' is the wordplay.
'i twice' can mean two replacements for 'i'.
'elaborated' indicates anagramming the letters (an elaborate version of the letters).
'gut'+'feel'+'london'+'i'+'i'='gutfeellondonii'
'gutfeellondonii' with letters rearranged gives 'LINE OF LONGITUDE'.

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I've seen this clue in The Independent.

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