Worn-out brother to understand key to have (6-4)

'worn-out' is the definition.
(synonyms)

'brother to understand key to have' is the wordplay.
I cannot quite see how this works, but
'brother' could be 'bro' (short for brother) and 'bro' is found within the answer.
'understand' could be 'ken' (I've seen this in another clue) and 'ken' is found in the answer.
'key' could be 'd' (musical key) and 'd' is found within the answer.
'have' could be 'own' (to have something is to own it) and 'own' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?