One doing terribly shows colour (6)
I believe the answer is:
indigo
'colour' is the definition.
(indigo is a kind of colour)
'one doing terribly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'terribly' indicates an anagram.
'doing' is an anagram of 'ndigo'.
'i'+'ndigo'='INDIGO'
'shows' acts as a link.
(Other definitions for indigo that I've seen before include "Deep-blue dye or colour" , "Violet-blue dye" , "Deep dark blue" , "Violet dye" , "type of dye" .)