Here's a challenge: find something to subtract from "hammer" which does not result in a word that has "gun" as a substring. I've been unsuccessful so far.
Gun related stuff works: bullet, holster, barrel
Other stuff that works: key, door, lock, smooth
Some words that result in "flintlock": violence, anger, swing, hit, impact
if I'm allowed only 1 something, I can't find anything either, if I'm allowed a few somethings, "hammer - wine - beer - red - child" will get you there. Guessing given that a gun has a hammer and is also a tool, it's too heavily linked in the small dataset.
Well that's easy, subtract "gun" :P
Bullet
hammer - red = lock
The word "gun" itself seems to work. Package this as a game and you've got a pretty fun game on your hands :)