What's milky when shaken? Drink from barley (4,6)

I believe the answer is:

malt whisky

I'm a little stuck... Click here to teach me more about this clue!

'drink from barley' is the definition.
'malt whisky' can be an answer for 'drink' (malt whisky is a kind of drink). I'm unsure of the 'from barley' bit.

'what's milky when shaken?' is the wordplay.
'what's milky' anagrams to 'MALT WHISKY'.
Though, I am not clear how this is indicated.

Can you help me to learn more?

(Other definitions for malt whisky that I've seen before include "Distilled spirit" , "nightcap" .)

I've seen this clue in The Times.

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