Unflagging (13)

I believe the answer is:

indefatigable

'unflagging' is the definition.
(synonyms)

This is all the clue.

(Other definitions for indefatigable that I've seen before include "Assiduous" , "Unremitting" , "Tireless" , "Persisting tirelessly" , "Persistent" .)

I've seen this clue in The Independent.

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