Model material (5)
I believe the answer is:
balsa
'model material' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for balsa that I've seen before include "Very light wood in a slab" , "Central American forest tree used to make floats, rafts, models etc" , "wood for raft" , "Lightweight timber" , "lightweight boat-builder" .)