Greek character isn't a bad user of keys (7)
I believe the answer is:
pianist
'user of keys' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe there's a link between them I don't understand?
'greek character isn't a bad' is the wordplay.
'greek character' becomes 'pi' (letter from the Greek alphabet).
'bad' indicates an anagram.
'isnt'+'a'='isnta'
'isnta' is an anagram of 'anist'.
'pi'+'anist'='PIANIST'
Can you help me to learn more?
(Other definitions for pianist that I've seen before include "One tickling the ivories" , "key man" , "Key expert" , "Ashkenazy or Lang Lang" , "Key worker," .)