Tireless (13)

I think it could be either:

indefatigable

inexhaustible

'tireless' is the definition.
(synonyms)

This is all the clue.

(Other definitions for indefatigable that I've seen before include "with no end of energy" , "Assiduous" , "Persisting tirelessly" , "Relentless" , "Unremitting" .)

I've seen this clue in The Times, The Guardian, the Family Time and The Telegraph.

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