Plumber is a noun.
In general, "master plumber" is not capitalized unless it is part of a title preceding a person's name or part of a specific organization's title. For example, you would write "John Smith, Master Plumber," but "John Smith is a master plumber." Always check specific style guides for variations.
The part of speech for this particular word is a noun.
part of speech
The part of speech for "answer" is a noun.
what part of speech is beneath
adverb
The part of speech for "explicit" is an adjective.
The part of speech for "lecturer" is a noun.
The part of speech for "rapid" is an adjective.
The part of speech of "came" is a verb.
The part of speech for "breezy" is an adjective.
The part of speech for "buried" is a verb.