have a brush for it in earnest

have a brush for it in earnest = will have a great battle for it?

If so, how "brush" can be used that way?

Context:

18. Narrative Continued by the Doctor: End of the First Day's Fighting

WE made our best speed across the strip of wood that now divided us from the stockade, and at every step we took the voices of the buccaneers rang nearer. Soon we could hear their footfalls as they ran and the cracking of the branches as they breasted across a bit of thicket.

I began to see we should have a brush for it in earnest and looked to my priming.

"Captain," said I, "Trelawney is the dead shot. Give him your gun; his own is useless."

They exchanged guns, and Trelawney, silent and cool as he had been since the beginning of the bustle, hung a moment on his heel to see that all was fit for service. At the same time, observing Gray to be unarmed, I handed him my cutlass. It did all our hearts good to see him spit in his hand, knit his brows, and make the blade sing through the air. It was plain from every line of his body that our new hand was worth his salt.
More:

http://www.online-literature.com/stevenson/treasureisland/18/