Colour supplement sent without its first article (7)
I believe the answer is:
magenta
'colour' is the definition.
(magenta is a kind of colour)
'supplement sent without its first article' is the wordplay.
'supplement' becomes 'mag' (short for magazine).
'without its first' means to remove the first letter.
'article' becomes 'a' (indefinite article).
'sent' with its first letter taken off is 'ent'.
'mag'+'ent'+'a'='MAGENTA'
(Other definitions for magenta that I've seen before include "Deep purplish red" , "this might be an ink" , "Reddish purple" , "Light purplish-red" , "Reddish-purple colour" .)