Not Ailanthus, wrong leaflet shape. Might be Butternut Walnut or one of the Asian walnuts - to check, cut a twig lengthways, and see if it has chambered pith (like this pic).
Thanks. Does it look like Black Walnut? I cut a branch and this is what it looks like. I'm concerned because I have conifers planted nearby.
Yes, a walnut for certain; could be Black Walnut, but I'm not convinced it is that particular species of walnut, the young foliage looks too 'soft and downy', a bit more like Butternut Walnut perhaps. I'd not be too worried about your new conifers, in my experience, juglone damage caused by walnuts is more talked of than seen.
Some pics later in the summer when the leaves have fully expanded and hardened should help (best post them in a reply to this thread, rather than starting a new thread).