ustvari

1. actually; in fact
   Izgleda strogo, ali je ustvari veoma dobar. — He looks strict, but he's actually very kind.
   Ustvari, nisam ni gladan. — Actually, I'm not even hungry.
2. (correcting oneself) or rather; I mean
   Doći ću u pet, ustvari u šest. — I'll come at five, or rather at six.

The standard spelling is two words, u stvari; ustvari is very common but is not the form prescribed by the Pravopis.