It's a four-letter word, aptly (4)
I believe the answer is:
oath
'it's a four-letter word aptly' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oath that I've seen before include "Vow or expletive" , "Curse - sworn statement" , "Solemn vow" , "Sworn declaration" , "A curse - or a pledge" .)