A mixture of substances decorate frock suitable for semi-formal occasions (8,5)

I believe the answer is:

cocktail dress

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'decorate frock suitable for semi-formal occasions' is the definition.
I can't judge whether this definition defines the answer.

'a mixture of substances' is the wordplay.
'a mixture' becomes 'cocktail'.
'of substances' becomes 'dress' (I can't explain this - if you can you should believe this answer much more).
'cocktail'+'dress'='COCKTAIL DRESS'

Can you help me to learn more?

(Other definitions for cocktail dress that I've seen before include "Black tie for ladies?" , "semi-formal wear" .)

I've seen this clue in The Irish Times.

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