Novel Thoreau finally had written (8)
I believe the answer is:
authored
'written' is the definition.
(authoring is a kind of writing)
'novel thoreau finally had' is the wordplay.
'novel' indicates an anagram.
'finally' suggests the final letters.
The final letter of 'had' is 'd'.
'thoreau' anagrammed gives 'authore'.
'authore'+'d'='AUTHORED'
(Other definitions for authored that I've seen before include "Wrote" , "written" .)