Courage (abstract noun) is a great virtue.
Though I am quite sure it cannot be considered as an adjective also, but just want to clear the doubt once and for all.Can a part of sentence be labelled under more than one part of speech?
No, abstract nouns and adjectives are two different things. Anger is an abstract noun, while "angry" is and adjective. "Length" is an abstract noun, while "long" is an adjective.
While specific words can be categorized into different parts of speech depending on how it is used in a sentence, specific parts of a particular sentence can only be categorized as one part of speech. For example, take the word "love".
A mother's love is eternal. (here love is an abstract noun) I love pizza. (here love is a verb)
Courage is not an adjective. A word can be more than one part of speech and you have to look at how the word works in a particular sentence . Let's take "can". In the following sentence it is a verb: "He can see the sea from his house".However, "I can't open this can of tuna fish", here it's a noun."His", is both an adjective and an adverb. "Italian", is both a noun' "I can speak Italian", and an adjective, "I just finished reading an Italian novel. Of course this is possible only in different sentences.