You can use some low level commands:
The spacing between “def” and “=” is set to one point, adapt it to your liking. The “=” is lowered by half the height of the math axis, which is available as
Personally, I don't like such a symbol. If I were forced to use something like “it is defined to be”, I'd probably choose
\coloneq, which is asymmetric. But I've never felt the mathematical need for such a symbol.