Oooops part was already guessed and red at that, I got confused with it again haha.
I think we can rule out horn (clue could have been alarm, sound or blare for example), dream (i'm sure there's something like "success" you could do), east (i think he could have thought of some explosion that happened in the east, like fukushima, krakatau or tunguska or something). Pass doesn't feel right also, he could have given a clue like "grenade" and I think it would have been better than giving a single word clue. I think even for bug there's something like "locusts" that would have worked as a two word clue, I think boom is such a crappy word you need to get creative with it and trust your guessers to understand.
That would leave us with leprechaun, mirror, or link.
Our clues:
avengers 4 - that was iron, comic, contract?, ?
Custard 3 -lemon, cream, ?
porcelain 2 - sink and ivory
So the last one is going to pertain to avengers and/or custard. Leprechaun and the avengers are both fictional? But if you know for a fact that Galzria would know that ant-man is an avenger (i wouldnt have), I think bug might be our best bet. Can't think of anything with mirror, I guess avenging is a LINK between the avengers but that seems too far-fetched to give a one word clue on the last turn. Can't connect custard with leprechaun or bug, maybe you could put bugs in a pie but sounds pretty out there as a clue.
So I would say it's either bug or leprechaun, and I'm leaning bug. Thoughts, please do ask if you can't follow my thought process somewhere, this was a stream of consciousness