Not exactly wily, but unctuous (4)

I believe the answer is:

oily

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'unctuous' is the definition.
(I know that unctuous can be written as oily)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for oily that I've seen before include "Smoothly ingratiating" , "Smooth, ingratiating" , "Unpleasantly ingratiating" , "Like a lot of fish" , "Smarmy" .)

I've seen this clue in the Evening Standard.

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