Tarnish dance with dirty look (9)
I believe the answer is:
discolour
'tarnish' is the definition.
Although both the answer and definition are verbs in their base form, I cannot see how one could define the other.
'dance with dirty look' is the wordplay.
'dance' becomes 'disco' (disco can mean a dance event).
'with dirty look' becomes 'lour' (I can't explain this - if you can you should believe this answer much more).
'disco'+'lour'='DISCOLOUR'
Can you help me to learn more?
(Other definitions for discolour that I've seen before include "fox?" , "Fade - stain" , "tarnished" , "remove shades" .)