One only, at the inside, you leave severely alone (7)

I believe the answer is:

isolate

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'alone' is the definition.
(thesaurus)

'one only at the inside' is the wordplay.
'one' becomes 'i' (Roman numeral).
'only' becomes 'sole' (synonyms).
'the inside' is an insertion indicator.
'sole' going around 'at' is 'solate'.
'i'+'solate'='ISOLATE'

'you leave severely' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.

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(Other definitions for isolate that I've seen before include "split up" , "Point out" , "Maroon" , "Segregate. close off" , "Cause to be alone" .)

I've seen this clue in the Evening Standard.

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