100 percent behind wood thwart (9)
I believe the answer is:
forestall
'thwart' is the definition.
(I've seen this before)
'100 percent behind wood' is the wordplay.
'f00 percent' becomes 'all' (I've seen this in another clue).
'behind' means one lot of letters go next to another.
'wood' becomes 'forest' (synonyms).
'all' after 'forest' is 'FORESTALL'.
(Other definitions for forestall that I've seen before include "Thwart" , "Avert" , "Arrive first" , "Prevent by taking advance action" , "Stop" .)