Headcanon 001 ➴ Nikolai's name
➵ Over the years of being with him, Clara has called Nikolai many-a-names, it's even gotten to a point where you can tell if she's pissed off at him or not.
When they're one a mission (and sometimes just in general): "Wolf Boy" or "Wolf"
the wolf spider is just to long, by the time she finishes saying that code name then a million civilians could be slaughtered.
When she's teasing him: "Nicky", "Nicky-poo", or "Russian Princess"
Really anything that will frustrate him--thus making her smile
When she's angry with him: "Nikolai"
Sharp and simple, kind of like a mother would use their child's full name if they'd done something wrong, except that when it's coming from Clara it's a little more scary.
But most the time when she's talking to or referring to Nikolai she just calls him "Nick" short and sweet.