Chance is quick to lose the head (4)
I believe the answer is:
risk
'chance' is the definition.
('risk' can be a synonym of 'chance')
'is quick to lose the head' is the wordplay.
I cannot really see how this works, but
'is' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for risk that I've seen before include "Expose to danger" , "Endanger, jeopardise" , "Gamble" , "Danger or hazard" , "dangerous thing" .)