Both couples in cast switched parts of play (4)

I believe the answer is:

acts

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

'parts of play' is the definition.
(I've seen this before)

'both couples in cast switched' is the wordplay.
'cast' anagrams to 'ACTS'.
But, I'm not clear how the anagram is indicated.

Can you help me to learn more?

(Other definitions for acts that I've seen before include "Fifth book of the New Testament" , "Goes" , "Takes steps" , "Cast in major play sections" , "Biblical book" .)

I've seen this clue in The Guardian.

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