King captured one sure to fail (5)
I believe the answer is:
goner
'sure to fail' is the definition.
I can't judge whether this defines the answer.
'king captured one' is the wordplay.
'king' becomes 'GR' (short for Georgius Rex, King George in Latin).
'captured' is an insertion indicator.
'gr' enclosing 'one' is 'GONER'.
Can you help me to learn more?
(Other definitions for goner that I've seen before include "One doomed to die" , "No-hoper" , "dying man" , "Someone as good as dead" , "One dead or beyond hope of recovery" .)