Feature of the French Revolution, being decapitated? Wrong (5)
I believe the answer is:
error
'wrong' is the definition.
(I've seen this before)
'feature of the french revolution being decapitated?' is the wordplay.
'feature of the french revolution' becomes 'terror' (as in the Reign of Terror).
'being decapitated?' means to remove the first letter.
'terror' with its first letter taken off is 'ERROR'.
(Other definitions for error that I've seen before include "Blunder, slip" , "Trial and ___" , "Slip, mistake" , "Miscalculation" , "Howler" .)