Are you asking "if I pull the trigger to drop the hammer on a chambered round, will it fire?" If so, the answer is "in all likelihood, if the gun isn't defective, yes."
I think you're assuming that you are holding the hammer with your other hand and decocking or lowering it manually with the trigger depressed--I think in that case it depends on the gun (does it have a firing pin block? grip safety?), but I would assume that if you let the hammer fall too fast, it could fire the round. What kind of gun are you talking about?