When you say "ur is indefinite", I guess you mean ur is NEUTER GENDER because it has "et" instead of "en" (as an indefinite article). Both et and en are indefinite articles.
In short; et & en are indefinite articles, det/den/de are definite articles.
If you are looking for an answer why it was "historiske" and not "historiskt" in this example:
When the article is definite (det= THE/THAT), no matter what the noun is, any adjective before is written as the plural form of that adjective -> det historiske ur (THE/THAT historical clock). Whereas if the article is indenifinite (et=A/AN) the adjective before the noun is formed according to the indefinite article et-> et historiskt ur (a historical clock).