Billy took forty winks - that's novel (9)
I believe the answer is:
kidnapped
'that's novel' is the definition.
(I've seen this before)
'billy took forty' is the wordplay.
'billy' becomes 'kid' (I can't justify this - if you can you should give a lot more credence to this answer).
'took forty' becomes 'napped' (I've seen this in another clue).
'kid'+'napped'='KIDNAPPED'
'winks' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for kidnapped that I've seen before include "Abducted" , "Classic novel" , "Book by Stevenson" , "Taken prisoner for ransom - R. L. Stevenson novel" , "novel for a boy" .)