Good morning gentlemen,
The OP states that the finish is cracked. If the crack runs through the finish to the surface of the wood, all the polishing in the world isn't going to make it go away.
Not a lot of info to go on here. "not a very expensive cue", "dent in the butt", "it cracked the finish".
Knowing the brand of cue would help to identify the finish that was used.
Is it UV, auto clear-coat, lacquer?
In any event, a CM or Cue-Tech should be consulted if you want it repaired correctly. A spot-finish repair may be all that's needed.
Nail polish or CA will indeed seal the damage but won't 'bite' the existing finish enough to make the cracked finish disappear. It will always be visible until the crack is sanded down to bare wood and refinished.
Without a pic, it's hard to advise further.
Dents/accidents happen. I doubt the 'perp' did it on purpose. Chances are that during the life of this cue, it may acquire a few more dents/dings. No Biggie.
If the cue has significant value to you, you may elect at some time to have the cue completely refinished.