Dye, one doing badly (6)
I believe the answer is:
indigo
'dye' is the definition.
(I know that indigo is a type of dye)
'one doing badly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'badly' indicates anagramming the letters (letters in the wrong order).
'doing' anagrammed gives 'ndigo'.
'i'+'ndigo'='INDIGO'
(Other definitions for indigo that I've seen before include "Blue-purple colour in spectrum" , "Deep-blue dye or colour" , "Blue/violet dye" , "Sixth rainbow colour" , "A deep blue dye or colour" .)