Nonebutthe 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