A chunk getting knocked over — it's wooden (5)
I believe the answer is:
balsa
'it's wooden' is the definition.
I can't tell whether this defines the answer.
'a chunk getting knocked over' is the wordplay.
'chunk' becomes 'slab' (I have seen 'Chunk of wood' mean 'slab' so perhaps 'chunk' could also mean 'slab').
'getting knocked over' is a reversal indicator.
'a'+'slab'='aslab'
'aslab' back-to-front is 'BALSA'.
Can you help me to learn more?
(Other definitions for balsa that I've seen before include "South American tree" , "light raft" , "Very light wood in a slab" , "Central American forest tree used to make floats, rafts, models etc" , "lightweight boat-builder" .)