Deep purplish-red (7)
(Other definitions for magenta that I've seen before include "Shade of purple" , "Bright purplish-red" , "Colour named after a bloody 1859 battle in Italy" , "Purplish-pink colour" , "Reddish-purple colour" .)
'deep purplish-red' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for crimson that I've seen before include "Blood-red" , "Bright red" , "Microns of colour" , "Flushed" , "Blush" .)