One doing badly shows colour (6)
I believe the answer is:
indigo
'colour' is the definition.
(indigo is a kind of colour)
'one doing badly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'badly' is an anagram indicator.
'doing' with letters rearranged gives 'ndigo'.
'i'+'ndigo'='INDIGO'
'shows' acts as a link.
(Other definitions for indigo that I've seen before include "Adulthood" , "Spectral colour" , "Tropical plant" , "supplier of blue stuff" , "Ellington's Mood" .)