Should I use a definite or indefinite article with a noun that refers to an object in the first time, but gets some clarification right after? For example, "This is a/the robot sent to settle the situation" or, a bit differently worded, "This is a/the robot that has been (was?) sent to settle down the situation".

On the one hand, an object is referred to the first time. On contrary, it is made defined by additional information. Which article should I use?

Question

Should I use a definite or indefinite article with a noun that refers to an object in the first time, but gets some clarification right after? For example, "This is a/the robot sent to settle the situation" or, a bit differently worded, "This is a/the robot that has been (was?) sent  to settle down the situation".

On the one hand, an object is referred to the first time. On contrary, it is made defined by additional information. Which article should I use?

Answer

You should use the definite article in your example because it's a very specific, unique robot and you're identifying it. You could use the indefinite article first if you changed the wording so that you're not talking about a specific robot, but just letting your listener know of its existence:

They've sent a robot to settle the situation. This the robot (that) they've sent.