Chief structure (4)
I believe the answer is:
arch
This clue is a double definition.
'chief' is the first definition.
(synonyms)
'structure' is the second definition.
(I know that arch is a type of skeletal structure)
(Other definitions for arch that I've seen before include "Chief - part of foot" , "Structural curve" , "Mischievous - curve" , "Supporting structure" , "part of fingerprint" .)