Her beauty derives from her grey matter (5)
I believe the answer is:
pearl
'her' is the definition.
'beauty derives from her grey matter' is the wordplay.
I cannot really see how this works, but
'matter' could be 'pea' (pea is a kind of matter) and 'pea' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for pearl that I've seen before include "Precious stone from shellfish" , "Gem from a bivalve" , "Small type" , "Gem from oyster" , "prized example" .)