Latin poet's wasted promise (9)
I believe the answer is:
potential
'promise' is the definition.
(I've seen this before)
'latin poet's wasted' is the wordplay.
'wasted' indicates an anagram.
'latin'+'poet'='latinpoet'
'latinpoet' anagrammed gives 'POTENTIAL'.
(Other definitions for potential that I've seen before include "Inherent capacity not realised" , "Latent, not yet actual" , "Capacity, ability to be developed" , "Inherent capacity for becoming something" , "Possibility, promise" .)