Stupidity of bridal vow to cherish one clearly gutless (6)
I believe the answer is:
idiocy
'stupidity' is the definition.
(idiocy is a kind of stupidity)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for idiocy that I've seen before include "Great folly" , "Stupid behaviour or talk" , "Chopper" , "Extreme silliness" , "Predatory seabird" .)