Former monarch and husband rejected hair dye (5)
I believe the answer is:
henna
'hair dye' is the definition.
(henna is a kind of hair dye)
'former monarch and husband rejected' is the wordplay.
'former monarch' becomes 'anne' (Queen Anne).
'and' means one lot of letters go next to another.
'husband' becomes 'h'.
'rejected' says the letters should be written in reverse.
'anne'+'h'='anneh'
'anneh' back-to-front is 'HENNA'.
(Other definitions for henna that I've seen before include "Red hair dye" , "Dye used in skin decoration" , "Vegetable dye" , "(Shrub producing) reddish dyestuff" , "Shrub used for reddish-brown dye" .)