Thwart made of wood, entirely (9)
I believe the answer is:
forestall
'thwart' is the definition.
(I've seen this before)
'wood entirely' is the wordplay.
'wood' becomes 'forest' ('forest' can be a synonym of 'wood').
'entirely' becomes 'all' (I've seen this before).
'forest'+'all'='FORESTALL'
'made of' acts as a link.
(Other definitions for forestall that I've seen before include "Arrive first" , "Hinder by anticipation" , "Obstruct by taking advance action" , "Anticipate" , "Take action before" .)