Unyielding state makes a Republican withdraw from group support (8)
I believe the answer is:
solidity
'unyielding state' is the definition.
(state of being solid or unyielding)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for solidity that I've seen before include "Substance" , "Firm quality" , "Firmness" .)