Tireless beating failed perhaps (13)
I believe the answer is:
indefatigable
'tireless' is the definition.
(synonyms)
'beating failed perhaps' is the wordplay.
'perhaps' indicates anagramming the letters.
'beating'+'failed'='beatingfailed'
'beatingfailed' anagrammed gives 'INDEFATIGABLE'.
(Other definitions for indefatigable that I've seen before include "Relentless" , "with no end of energy" , "Persistent" , "Tireless" , "Unremitting" .)