Spooner says cad's loud music is why he can't start his book (7)
'spooner says' is the definition.
Although both the answer and definition are plural nouns, I don't see how they can define each other.
'cad's loud music is why he can't start his book' is the wordplay.
I cannot really see how this works, but
'music' could be 'te' and 'te' is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for writers that I've seen before include "Authors" , "Bookmakers" , "Scribes" .)