Solemn vow (4)
I believe the answer is:
oath
'solemn vow' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oath that I've seen before include "gadzooks!" , "Solemn vow" , "Curse or solemn word" , "Swear word" , "Strong language" .)