Question

How do I draw a standalone arrow?

Answer 1

Create two nodes, connect the nodes with the desired edge, and finally make the two nodes invisible (by chosing "No Color" as fill color and line color).

yEd is a graph editor and by definition a graph cannot contain "standalone arrows".

Comments

This makes no sense, what is a node. yEd might be easy for the developers, but BRUTAL for us users

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It's only just names... yEd uses the terms "node" and "edge" to refer to an "object" and a "connection line," respectively.
The terminology stems from graph theory (mathematics); but it's not dangerous at all ;-).

(Since there are so many different names one can think of instead of "node" and "edge" -- object, rectangle, box, element, vertex, point, etc. and arrow, connection, connection line, line, arc, etc. respectively -- yEd just sticks to the original ones.)