Useful plant, one doing badly (6)
I believe the answer is:
indigo
'useful plant' is the definition.
(plant used to make dyes)
'one doing badly' is the wordplay.
'one' becomes 'I' (Roman numeral).
'badly' indicates anagramming the letters.
'doing' anagrammed gives 'ndigo'.
'i'+'ndigo'='INDIGO'
(Other definitions for indigo that I've seen before include "Blue/violet dye" , "Violet-blue dye" , "I of ROYGBIV" , "A deep blue dye or colour" , "type of dye" .)