Assert casting upset gripping performer (7)

I believe the answer is:

actress

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

'performer' is the definition.
(actress is a kind of performer)

'assert casting upset gripping' is the wordplay.
'ACTRESS' is hidden backwards in amongst the letters.
However, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for actress that I've seen before include "Stage performer" , "one with a particular role?" , "Female player" , "Female stage-player" , "Casters (anag.)" .)

I've seen this clue in The Telegraph.

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