Exhortation to be unflagging? (8)

I believe the answer is:

tireless

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'to be unflagging?' is the definition.
('tireless' can be a synonym of 'unflagging')

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(Other definitions for tireless that I've seen before include "Having inexhaustible energy" , "Full of energy" , "Permanently lively" , "Never wearying" , "Indefatigable, unflagging" .)

I've seen this clue in the Evening Standard.

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