Blue extremes starting off clean (5)
I believe the answer is:
azure
'blue' is the definition.
(I know that azure is a type of blue)
'extremes starting off clean' is the wordplay.
'extremes' becomes 'az' (extreme letters of the alphabet).
'starting off' means to remove the first letter (the starting letter is removed).
'clean' becomes 'pure' (similar in meaning).
'pure' with its first letter removed is 'ure'.
'az'+'ure'='AZURE'
(Other definitions for azure that I've seen before include "Shade of blue" , "tone perhaps" , "Light blue" , "Sky-coloured" , "Deep sky-blue colour" .)