Start the work if you can. Acknowledge the grant if you get it.
If you don't have the necessary resources to start the proposed research without the new grant, the only thing you can do is wait for the new grant. Yes, your idea could become obsolete after six months; in fact, your might already be obsolete. Nothing you can do about that. Let it go.
If you do have the necessary resources to start the proposed research without the new grant, go right ahead. (Those necessary resources definitely include your time and expertise; they might also include specialized equipment, money for research assistants, money for publication costs, money for travel, ethics board approval, and/or other things I didn't think of.)
But while you're waiting for the grant decision, remember that developing a research idea into a publication also takes several months. You have to do the actual research; you have to write the paper; the paper has to be reviewed and edited. All of these steps take significant time.
If you can get a paper all the way to print before the proposal decision, you don't need to acknowledge the new grant, because it doesn't exist yet. But this seems rather unlikely; any idea that you can get to publication in only a few months is a weak basis for a grant proposal.
On the other hand, if the grant comes in before your paper is actually published, pat yourself on the back and then acknowledge the grant. Even if you are already revising the galley proofs. ("Research by this author is partially supported by XXX under grant ZZZ.") You have nothing to lose by being generous to the funding agency.