The value of an Isle on the Thames (5)

I believe the answer is:

worth

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

I don't understand how the rest of the clue works.

(Other definitions for worth that I've seen before include "Value (of something)" , "Merit - value" , "Throw (anag.)" .)

I've seen this clue in the Evening Standard.

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