Model airplane material (5)
I believe the answer is:
balsa
'model airplane material' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for balsa that I've seen before include "Tree with light soft wood" , "Central American forest tree used to make floats, rafts, models etc" , "A slab (anag.)" , "Lightweight timber" , "Wood used by model makers" .)