Material to oppose the structure, essentially (7)
I believe the answer is:
buckram
'material' is the definition.
(buckram is a kind of material)
'oppose the structure essentially' is the wordplay.
'oppose' becomes 'buck' (bucking is a kind of opposing).
'the structure essentially' becomes 'ram' (I can't explain this - if you can you should give a lot more credence to this answer).
'buck'+'ram'='BUCKRAM'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for buckram that I've seen before include "Coarse stiffened cloth" , "Coarse material" , "used in bookbinding" , "Bookbinding material - cub mark (anag)" , "stiff material" .)