Eternal task diverting a venomous reptile (11)
I believe the answer is:
rattlesnake
'a venomous reptile' is the definition.
(type of venomous snake)
'eternal task diverting' is the wordplay.
'diverting' is an anagram indicator (letters diverted elsewhere).
'eternal'+'task'='eternaltask'
'eternaltask' anagrammed gives 'RATTLESNAKE'.
(Other definitions for rattlesnake that I've seen before include "Poisonous (and noisy!) reptile" , "Venomous creature" , "Potentially lethal reptile" , "Serpent with noisy tail" , "Virulent type" .)