It's a four-letter word, aptly (4)
I believe the answer is:
oath
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">it's a four-letter word aptly</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)