Supportive material, strong (9)
I believe the answer is:
strapping
This clue might be a double definition.
'supportive material' is the first definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?
'strong' is the second definition.
(synonyms)
Can you help me to learn more?
(Other definitions for strapping that I've seen before include "Robust" , "Brawny" , "Burly" , "Tying up" , "stalwart" .)