Is an island river (4)

I believe the answer is:

isis

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

'is an island' is the wordplay.
'an island' becomes 'is'.
'is'+'is'='ISIS'

(Other definitions for isis that I've seen before include "Oxford river; goddess" , "Egyptian goddess and jihadist group" , "divinity" , "Repetitive Egyptian goddess" , "Egyptian goddess - Thames at Oxford" .)

I've seen this clue in the Evening Standard.

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