1) You want a dust explosion.
There are lots of materials that, if you have a pile of them or if they are at all wet, then they won't burn very well at all. Consider ordinary wheat flour, for example. A pile of flour will work quite well to extinguish small flames. And you don't need to be concerned about having many 10's of kg of flour in your kitchen close to your stove. It's less flammable than, for example, paper.
But toss it into the air as a fine powder, and suddenly you've got quite the amazingly explosive material. Such that "disappearing flour mill" is actually a thing. They need to take quite drastic steps to prevent the dust from hanging in the air, and from the small amount that is unavoidable ever getting an ignition source.
So a critter with a dry pouch full of something similar to flour, and a small ignition source, could in essence have a one-shot flame thrower. Blow the dust then ignite it. That would get the attention of a lot of potential predators. In the right circumstances it could be used to do a lot of destruction.
2) You want gun cotton.
Nitrocellulose, or gun cotton, is made from nitric acid and cellulose.
The reaction is fairly easy to produce, and isn't particularly difficult to control as long as you keep any ignition source well away.
Nitric acid is produced in small quantities by many organisms. So it is reasonable that it might be produced just as fast as it was used by the reaction, keeping the net concentration low.
Nitrocellulose is even a potentially useful building material for the bomb itself. It's quite hard and "springy." It can be used (with some chemicals added to reduce the explosive nature) to make such things as billiard balls. This meant that, before the anti-explosive was added, disappearing billiard halls were occasionally a thing. You see, it was once the habit for the winner of the game to stub out his cigarette on the cue ball. Boom!