Tarted up multicoloured clothes couturier's assistant sent back (10)
I believe the answer is:
prettified
'couturier's assistant sent back' is the definition.
I can't tell whether this definition defines the answer.
'tarted up multicoloured clothes' is the wordplay.
'tarted' becomes 'fitter' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'multicoloured' becomes 'pied' (synonyms).
'clothes' means one lot of letters goes inside another (some letters clothe or cover others).
'fitter' written backwards gives 'rettif'.
'rettif' put within 'pied' is 'PRETTIFIED'.
Can you help me to learn more?
(Another definition for prettified that I've seen is " given facelift".)