Smooth as a mass of oil (5)

I believe the answer is:

slick

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This might be a double definition.

'smooth' is the first definition.
(I know that slick is a more specific form of the action smooth)

'a mass of oil' is the second definition.
'slick' can be an answer for 'oil' (thesaurus). I am not sure about the 'a mass of' bit.

'as' acts as a link.

Can you help me to learn more?

(Other definitions for slick that I've seen before include "Unwanted floater" , "Smooth" , "it's very oily" , "Glib; oil spill" , "Sleek - film of oil" .)

I've seen this clue in The Daily Express.

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