None but the brave deserve the fair. The underlined word is a/an-

But can be used in the following ways:As a conjunction (connecting two phrases or clauses):She’s 83 but she still goes swimming every day.As a way of starting a new sentence and connecting it to the previous sentence:It was in Cairo that he met Nadia. But that’s another story.As a preposition (followed by a noun):None but the brave deserve the fair.There’s been nothing but trouble since he came.As an adverb:We can but hope that things will improve.Source: Macmillan Dictionary