An advocate of the principles of monarchy WordNet 3.0 & Open English WordNet

A royalist supports a particular monarch as head of state for a particular kingdom, or of a particular dynastic claim. Wikipedia

Support of a particular monarch as head of state for a particular territory Wikidata

An advocate of monarchy. OmegaWiki

An advocate of, or believer in, monarchy. Wiktionary

Impassioned allegiance to or advocacy of the establishment, maintenance, and/or interests of a particular king, royal house, or kingly dynasty; sometimes extended to the same of a non-royal (i.e., grand ducal, imperial, or other) family or sovereign; often contrasted with monarchism. Wiktionary

Advocacy of a king or royal house. Wiktionary (translation)

A legitimist, a supporter of a particular royal line, especially one in danger of being dispossessed of a throne or actually dispossessed of such, and claiming to have the better claim to the throne on the basis of line of descent; especially. Wiktionary