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Artists' medium with colours which can sharpen with a little effort (9)

Ross

I believe the answer is:

oilstones

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'can sharpen with a little effort' is the definition.
I can't tell whether this definition defines the answer.

'artists' medium with colours' is the wordplay.
'artists' medium' becomes 'oils' (I can't explain this - if you can you should believe this answer much more).
'with' means one lot of letters go next to another.
'colours' becomes 'tones' (toning is a kind of colouring).
'oils'+'tones'='OILSTONES'

'which' acts as a link.

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