Great fear of losing head in mistake (5)
I believe the answer is:
error
'mistake' is the definition.
(I know that mistake can be written as error)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for error that I've seen before include "Slip-up" , "Miscalculation" , "Boo-boo" , "Faulty judgment" , "Fallacy, mistake" .)