The only possibility is to lose (4)

I believe the answer is:

sole

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

'possibility is to lose' is the wordplay.
'lose' anagrams to 'SOLE'.
Though, I am unsure how this is indicated.

Can you help me to learn more?

(Other definitions for sole that I've seen before include "swimmer" , "A lone sort of fish - lose it" , "Kind of fish - lose it" , "Flatfish; exclusive" , "Underside of footwear" .)

I've seen this clue in the Evening Standard.

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