Ape and lion come to like milk (7)

I believe the answer is:

opaline

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'milk' is the definition.
Both the definition and answer are adjectives. Maybe there's an association between them I don't understand?

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

Can you help me to learn more?

(Other definitions for opaline that I've seen before include "of stones" , "Like some gemstones" , "whitish glass" , "Semi-translucent glass" , "Translucent glass" .)

I've seen this clue in The Globe and Mail.

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