A picture would indeed help. If the crack is right at the knot, it may be very difficult to stabilize, because of the resin/pitch in the knot.
I have had some luck stabilizing small hairline cracks using cyanoacrylate adhesives (such as Hot Stuff).
CA is usually available in 3 consistencies -- thin, medium, and thick.
I have been successful with the kind of crack that can be opened up a bit by bending the wood one way, then closed when bending the wood the other way. I open the crack just a hair and squirt in thin CA; if it disappears immediately, I squirt in medium. I then close the crack -- CA should squeeze out -- and clamp it closed. I usually follow with a squirt of accelerator to make sure the CA kicks. Then sand off the CA on the surface. If the cracked wood will not be seriously stressed, this has resulted in a virtually invisible repair. Sometimes thin epoxy can be used instead of CA with this method.
If a crack cannot be closed and is very small, and is in a place where the weakness resulting from the crack will not matter, it can sometimes be filled with epoxy. Using a slow setting epoxy, fill the crack (push in with a splinter or razor knife blade), watch for a while to see if additional filling is needed because the epoxy works into the crack by capillary action or gravity (this is why slow epoxy is used -- such sinking in can take a bit of time). When the epoxy is cured, it can be scraped or sanded even with the surface of the wood. The repair will be more or less visible, depending on its size and location, but it will prevent dirt or other material from getting into an open crack and making it worse.
A photo of your crack would help determine either of these techniques might be appropriate.