Fashionable girl has to leave one colour (6)
I believe the answer is:
indigo
'one colour' is the definition.
(I know that indigo is a dye)
'fashionable girl has to leave' is the wordplay.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'girl' becomes 'Di' (short for Diane or Diana).
'has' says to put letters next to each other.
'to leave' becomes 'go' (both can mean to depart).
'in'+'di'+'go'='INDIGO'
(Other definitions for indigo that I've seen before include "Spectral colour" , "Mood - (Duke Ellington)" , "Adulthood" , "supplier of blue stuff" , "Blue dyestuff" .)