Slowcoach tires too easily (8)
I believe the answer is:
tortoise
'slowcoach' is the definition.
(tortoises are slow animals)
'tires too easily' is the wordplay.
'easily' is an anagram indicator.
'tires'+'too'='tirestoo'
'tirestoo' with letters rearranged gives 'TORTOISE'.
(Other definitions for tortoise that I've seen before include "Would this animal turn to Tories?" , "Slow-moving land reptile" , "Expected to be slow" , "Shelled creature" , "Slow mover" .)