Chief supporter (4)
I believe the answer is:
arch
This might be a double definition.
'chief' is the first definition.
(synonyms)
'supporter' is the second definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
(Other definitions for arch that I've seen before include "Structure with a keystone" , "Curved gateway" , "Masonry construction for spanning an opening" , "Curved bony structure in the foot" , "Bridge span" .)