Material for a girl's mum? (6-2-5)
I believe the answer is:
mother-of-pearl
I believe this clue is a double definition.
'material' is the first definition.
(I have seen 'iridescent material' mean 'mother of pearl' so perhaps 'material' could also mean 'mother of pearl')
'girl's mum?' is the second definition.
(I've seen this before)
'for a' acts as a link.
(Other definitions for mother-of-pearl that I've seen before include "Ornamental material" , "Nacre" , "girl's mum" , "Iridescent material used in ornamentation" , "Decorative substance got from certain molluscs" .)