Perseverance (13)

I think it could be either:

determination

assiduousness

'perseverance' is the definition.
(I've seen this before)

This is the entire clue.

(Other definitions for determination that I've seen before include "Decision process" , "Willpower" , "legal decision" , "Firm resolve" , "Resolution" .)

I've seen this clue in The Times.

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