Chaste girl, one protected by mother, supplies element (9)
I believe the answer is:
magnesium
'element' is the definition.
(magnesium is a kind of element)
'chaste girl one protected by mother' is the wordplay.
'chaste girl' becomes 'agnes' (St Agnes is the patron saint of chastity).
'one' becomes 'i' (Roman numeral).
'protected by' means one lot of letters goes inside another.
'mother' becomes 'mum' (mum is a kind of mother).
'agnes'+'i'='agnesi'
'agnesi' going into 'mum' is 'MAGNESIUM'.
'supplies' is the link.
(Other definitions for magnesium that I've seen before include "Metal element, Mg" , "flammable element" , "Silvery-white metallic element" , "Silver-white metallic element" , "Silver-white metal" .)