This answer is a four-letter word (4)
I believe the answer is:
oath
'this answer is a four-letter word' 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 "'Profane, or solemn expression (4)'" , "Profane or obscene expression" , "darn, for example" , "Solemn vow" , "A curse - or a pledge" .)