A bit of colour inserted as hair treatment (5)
I believe the answer is:
rinse
'hair treatment' is the definition.
(I've seen this in another clue)
'a bit of colour inserted as' is the wordplay.
'a bit of' says the answer is hidden in the clue.
'RINSE' can be found hidden inside 'colour inserted as'.
(Other definitions for rinse that I've seen before include "Remove soap with clean water" , "Hair colourant" , "Hair tint" , "Flush" , "Cleanse in water" .)