Own (up)

The answer to this crossword question is the word “fess“. The clue “own (up)” is a wordplay, where “fess” is a homophone of the word “confess” or “own up“. “Own up” means to confess or admit to something, and a homophone is a word that sounds the same but has a different meaning. So, “own (up) is fess” is saying that the word for confessing or admitting is “fess“. In heraldry, a “fess” is also a horizontal band across the middle of a shield or coat of arms, but that definition isn’t relevant in this context.