A part for actress Thora (5)

I believe the answer is:

third

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'a part' is the definition.
(fraction)

I cannot understand how the remainder of the clue works.

(Other definitions for third that I've seen before include "Bronze medal place" , "Ordinal number" , "Ruthless interrogation" , "33 per cent" , "Bronze prize position" .)

I've seen this clue in the Evening Standard.

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