A runner can be one (4)

I believe the answer is:

bean

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'a runner can' is the definition.
The definition and answer can be both body parts as well as being singular nouns.
I also know that 'can' relates to this answer.

'be one' is the wordplay.
'one' becomes 'an' (an apple is one apple).
'be'+'an'='BEAN'

Can you help me to learn more?

(Other definitions for bean that I've seen before include "popular character with kids" , "Haricot seed" , "Kidney-shaped seed as for coffee" , "Nut" , "Seed; head" .)

I've seen this clue in the Evening Standard.

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