Clean a Dirty Maple Fretboard?

It might not be all dirt. Raw maple reacts with either light, or atmosphere, (not sure which, probably both), and turns grey over time.

Naphtha, (lighter fluid) is a strong solvent that won't damage nitro or polyurethane if used as a cleaner on a rag, and not saturated or exposed to the finish for an extended period of time.

Oxalic acid is used to bleach wood, but it might not restore the original natural maple hue, (which you might restore with stain). There are other methods to bleach wood.

Post some photos.

 

A picture would help