Not a mind prepared to be masterful (8)
I believe the answer is:
dominant
'masterful' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I can't see?
'not a mind prepared' is the wordplay.
'prepared' is an anagram indicator.
'not'+'a'+'mind'='notamind'
'notamind' anagrammed gives 'DOMINANT'.
'to be' is the link.
Can you help me to learn more?
(Other definitions for dominant that I've seen before include "a key" , "Most important or influential" , "In a controlling position" , "alpha male?" , "Most influential; fifth of scale" .)