Unfading colour always seen on top (9)
I believe the answer is:
evergreen
'unfading' is the definition.
(thesaurus)
'colour always seen on top' is the wordplay.
'colour' becomes 'green' (greening is a kind of colouring).
'always seen' becomes 'ever' (I am not sure about this - if you are sure you should believe this answer much more).
'on top' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'green' put after 'ever' is 'EVERGREEN'.
Can you help me to learn more?
(Other definitions for evergreen that I've seen before include "Scots pine, perhaps" , "Non-deciduous tree - always immature?" , "Star anise, say" , "box, perhaps" , "Holly, say" .)