Colourful line in underwear (6)
I believe the answer is:
paints
'colourful line in underwear' is the definition.
I can't judge whether this definition defines the answer.
'line in underwear' is the wordplay.
'line' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'underwear' becomes 'pants' (underpants).
'i' inserted within 'pants' is 'PAINTS'.
Can you help me to learn more?
(Other definitions for paints that I've seen before include "Makes picture with canvas and oils" , "Liquid pigments" , "Decorative coatings" , "Applies colour" .)