Colouring agent used by mother? (7)
I believe the answer is:
magenta
'colouring' is the definition.
(magenta is a kind of colouring)
'agent used by mother?' is the wordplay.
'used' is an anagram indicator.
'by' is a charade indicator (letters next to each other).
'mother?' becomes 'ma' (Ma is another word for mother or mum).
'agent' with letters rearranged gives 'genta'.
'genta' put after 'ma' is 'MAGENTA'.
(Other definitions for magenta that I've seen before include "French victory" , "fuchsia" , "Bright purplish-red" , "Light purplish-red" , "puce" .)