Are they subtly stirred into fast food? (4)

I believe the answer is:

fats

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'are they' is the definition.
The definition suggests a plural noun which matches the answer.

'subtly stirred into fast food?' is the wordplay.
'fast' can be anagrammed to 'FATS'.
Though, I'm not sure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for fats that I've seen before include "Lipids" , "Eg Waller or Domino" .)

I've seen this clue in the Evening Standard.

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