Brief threnodies sorted one out (9)
I believe the answer is:
shortened
'sorted one out' is the definition.
Both the definition and answer are adjectives. Maybe there's a link between them I don't understand?
'brief threnodies' is the wordplay.
'brief' becomes 'short' (similar in meaning).
'threnodies' becomes 'ened' (I can't explain this - if you can you should give a lot more credence to this answer).
'short'+'ened'='SHORTENED'
Can you help me to learn more?
(Another definition for shortened that I've seen is " Truncated".)