One doing work for Violet (6)
I believe the answer is:
Here is my best explanation:
'violet' is the definition.
(indigo is a kind of violet)
'one doing work' is the subsidiary indication.
'one' becomes 'i' (roman numeral).
'work' indicates anagramming the letters.
'doing' is an anagram of 'ndigo'.
'for' is the link.
(Other definitions for indigo that I've seen before include "Deep blue dye or colour", "I of ROYGBIV", "Settlement; New Mexico native", "A deep blue dye or colour", "Sixth rainbow colour", "Violet dye", "Plant dye", "Blue-violet colour", "Violet-blue dye" and "Deep-blue dye or colour".)