Artist diplomat is serious about? (7)

I believe the answer is:

matisse

'artist' is the definition.
(Henri Matisse)

'diplomat is serious about?' is the wordplay.
'about?' says the answer is hidden in the clue (I've seen 'about' mean this).
'MATISSE' can be found hidden inside 'diplomat is serious'.

(Other definitions for matisse that I've seen before include "Henri -, Fr. artist" , "Is mates turned on the French painter" , "Henri ---, Fauvist painter" , "Leading fauvist painter, d. 1954" , "Henri ---, Twentieth Century painter" .)

I've seen this clue in The Telegraph.

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