A lot of it depends on how deep you go and how much you decide to "borrow." Let's say you like Final Fantasy's aspect of summon magic, where you call in different monsters and creatures of legend to attack your enemies as spells before they are dismissed. But, you don't want to steal the idea in a way that Final Fantasy fans will chew you out for. The trick is to take the idea as a catalyst and change almost everything about it except for the very core component that you like.
So for example, instead of calling in creatures as spells (which only certain characters can do in most Final Fantasy games), perhaps each person has a mythical creature inside them that represents their soul and spirit. And maybe instead of a spell, people can call forward this spiritual being by means of an artifact such as a special ring or armband, or the summoner has to do something a bit more on the macabre side, like shed a bit of their own blood while focusing their thoughts or speaking an incantation. And instead of using the same summons that Final Fantasy uses, you decide to use your own with names you made, such as a sun dragon that goes by the name of Sytranix, or a massive steel golem called Jagharal.
After that, you decide to write your own "rules" for summoning, different from what Final Fantasy dictates. In the universe you're creating, anyone can summon, but they need to discover who or what their spiritual creature is first. And maybe that creature forces that person to pass some kind of test first before they would be willing to aid that person. Lastly, the person and their summoned creature form a bond, and if something happens to one, it happens to the other.
So, in this case, Final Fantasy inspired you, but you changed so much about the concept that a Final Fantasy player would be pretty hard-pressed to tell and prove it was exactly Final Fantasy's summon magic that inspired this idea. When you strip things down to very core components and you change everything else, you're not stealing. Stealing would be using the exact same idea, all of its aspects, using it the same way, and proving you did nothing original with it.
Every artist and writer gets inspired by something, but you need to take advantage of the opportunity to bend and totally reshape it into something unique and different.