Only "apply"? It does have that meaning so it's not wrong (as in 応用 applied, like applied physics, applied computer science). But if you need one broad meaning to mentally associate with this kanji, I think it would be "response".
yeah that's one of the many problems people have with Heisig
he omits lots of meaning and simplifies stuff so it's easier to remember
While I cannot say whether the whole Heisig RTK thing is helpful or not, if you read the introduction of the book then it says that the whole point is that you pick a single distinct English word for each kanji. It's not supposed to help you with learning the precise meaning of any kanji in the first place, so I always found that a weird thing to criticize. What, the run-of-the-mill readers you bought at the gas station for $5 don't give you perfect vision in every scenario? Yeah no shit, doesn't mean that there's no point to them being available.
Anyway I don't advocate for Heisig at all (not because I think it's bad or good, I just don't really care about anyone's learning method), but if you're gonna go down that route, read up about the method beforehand and do it properly is my recommendation. Heisig's intention is not teaching you a perfect meaning for every kanji out there, the idea is that even though you don't know any Japanese vocabulary, you still can associate every kanji with something, that something being a single English word that most of the time kinda goes hand in hand with how the kanji is often used. So when you come across the kanji in the wild you can recognize it and differentiate it from others. That's also why there's hardly a point in doing Heisig if you already have a sizable Japanese vocabulary, because at that point you can just use an actual Japanese word that the kanji is used in to build that kind of association. English is just used because it's assumed that you already know that language.
応 in Remembering Kanji 1 (James W. Heisig)
In this book author suggest to remember this kanji as "apply". Does anyone know why?