heir - Definitions from Dictionary.comHeir apparent (1375) has the Fr. order of noun-adj.
The noun heir is short for heir apparent, from the proper noun Heir apparent, which is why it doesn't require an article; e.g. she is (the / an) heir to the thrown.be (linking verb) + heir (noun) to (preposition)______________
Note, as for being a verb, well,heir, verb (used with object)
5. Chiefly South Midland and Southern U.S. to inherit; succeed to.