Cooked quince alone isn't valueless (15)
I believe the answer is:
inconsequential
'valueless' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I don't see?
'cooked quince alone isn't' is the wordplay.
'cooked' indicates anagramming the letters (letters cooked into a new form).
'quince'+'alone'+'isnt'='quincealoneisnt'
'quincealoneisnt' anagrammed gives 'INCONSEQUENTIAL'.
Can you help me to learn more?
(Other definitions for inconsequential that I've seen before include "unrelated to the foregoing" , "Trivial" , "Petty" .)