Know it will fetch, if you sell it (7)

I believe the answer is:

realise

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'know' is the definition.
(I know that know is a more specific form of realise)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for realise that I've seen before include "Grasp" , "Tumble" , "Understand - cash in" , "catch on" , "Be fully aware of or convert plan into fact" .)

I've seen this clue in the Evening Standard.

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