Pure he may be, but not necessarily simple (5)

I believe the answer is:

simon

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'pure he' is the definition.
'Simon' can be an answer for 'he'. I am not sure about the 'pure' bit.

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

Can you help me to learn more?

(Other definitions for simon that I've seen before include "he says it's a game" , "- Bol'var, S. American liberator" , "Bolivar, de Montfort, Callow or Cowell?" , "Peter (NT)" , "Harris and Coveney" .)

I've seen this clue in the Evening Standard.

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