Turn over a thick plank, possible surfboard material (5)
I believe the answer is:
balsa
'material' is the definition.
(balsa is a kind of material)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for balsa that I've seen before include "Something light" , "Tree yielding light wood" , "Tree with light soft wood" , "Strong lightweight wood" , "lightweight boat-builder" .)