Rapid digestion of fats (4)

I believe the answer is:

fast

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

'rapid' is the definition.
(similar in meaning)

'digestion of fats' is the wordplay.
'fats' anagrams to 'FAST'.
Though, I'm not clear how the anagram is indicated.

Can you help me to learn more?

(Other definitions for fast that I've seen before include "Quick; firmly fixed" , "Rapid; abstain from food" , "Quickly; tight" , "Renounce food; quick" , "requiring only brief exposure" .)

I've seen this clue in the Evening Standard.

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