In short, is he the greatest? (3)

I believe the answer is:

max

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'in' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

'short is he the greatest?' is the wordplay.
'short' means to remove the first letter.
'is he' becomes 'em'.
'the greatest?' becomes 'ax' (I am not sure about this - if you are sure you should believe this answer much more).
'em' with its first letter removed is 'm'.
'm'+'ax'='MAX'

Can you help me to learn more?

(Another definition for max that I've seen is " The greatest".)

I've seen this clue in the Evening Standard.

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