kind of lemon tree
櫞 refers specifically to a type of lemon tree, a citrus plant bearing sour fruit.
櫞 combines the tree radical 木 with 緣, which likely provides the sound エン. The character was created to name a specific citrus tree.
The tree radical 木 on the left tells you this is a plant. The right side 緣 looks tangled like vines, but here it represents a lemon tree with its branches and fruit.
For エン, picture the end of a branch where a lemon hangs: end -> エン, and the tree 木 holds the fruit.
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citron (Citrus medica)