Robin Hood's friend is potty? (6,4)

I believe the answer is:

little john

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'robin' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see a link between them that I can't see?

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

Can you help me to learn more?

(Other definitions for little john that I've seen before include "big man in 7 [FOREST]" , "Outlaw" , "Associate of 8 10" , "One of Robin Hood's followers" , "Robin Hood's giant follower" .)

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