Close to a lengthy perimeter... (9)

I believe the answer is:

alongside

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'close to' is the definition.
(I've seen this before)

'a lengthy perimeter' is the wordplay.
'lengthy' becomes 'long' (synonyms).
'perimeter' becomes 'side' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'long'+'side'='ALONGSIDE'

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(Other definitions for alongside that I've seen before include "next door" , "Next to - alien dogs (anag)" , "Parallel to" , "Aligned so as to be close to pier" , "Juxtaposed" .)

I've seen this clue in The Irish Times.

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