Risky sacrificing a rook that's valuable (8)
I believe the answer is:
precious
'that's valuable' is the definition.
(similar in meaning)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for precious that I've seen before include "Very fastidious" , "Much loved" , "Valuable, treasured" , "Of high worth or cost" , "Cherished or of high worth" .)