Nominally, she's not without grey matter (5)

I believe the answer is:

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'grey matter' is the definition.
'pearl' can be an answer for 'grey' (thesaurus). I'm unsure of the 'matter' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for pearl that I've seen before include "Object of great worth" , "something highly valued" , "Drop" , "finest example" , "Gem; size of type" .)

I've seen this clue in the Evening Standard.

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