Upset friend has left: brilliant mathematician (7)

I believe the answer is:

laplace

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'mathematician' is the definition.
(I know that Laplace is a mathematician)

'upset friend has left brilliant' is the wordplay.
'upset friend has' becomes 'lap' (I can't justify this - if you can you should believe this answer much more).
'left' becomes 'l' (common abbreviation).
'brilliant' becomes 'ace' (ace can informally mean very good).
'lap'+'l'+'ace'='LAPLACE'

Can you help me to learn more?

(Other definitions for laplace that I've seen before include "French astronomer and mathematician" , "French scientist" , "Pierre-Simon -, Fr. mathematician, astronomer" .)

I've seen this clue in The Guardian.

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