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 definition and answer are singular nouns.
Perhaps there's an association between them I don't understand?

I cannot understand how the rest 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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