Thoreau's mentor (7)
I believe the answer is:
emerson
'thoreau's mentor' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for emerson that I've seen before include "Ralph Waldo ......., 19th century American transcendentalist" , "Ralph Waldo --, US poet" , "Ralph Waldo . . . . . . ., wrote The American Scholar" , "Ralph Waldo --, US author" , "American author, 1803-82" .)