Hark! the herald angels sing, -

Is 'herald' a noun, verb, adjective or funky old english usage.

This is a great question.

It is feasible that it's a noun and belongs with angel as part of a compound noun. Therefore, it could be written herald-angel (a bit like driver-mechanic).

However, in English, many nouns are used as adjectives (e.g. fish face, car wheel, house brick), and it is likely that herald is an adjective formed from the noun. That would be my guess.

Happy to be corrected.