Is the following sentence definite or indefinite relative clause?

The default reading would be that of a non-restrictive relative clause since it provides additional temporal information ("I met my uncle. By the way I did so when he had just arrived and not later"). However, in this case "which" should be preceded by a comma to signal this import. A restrictive reading without a comma could be yielded as well in case you referred to the only uncle who arrived among others who were also supposed to arrive but didn't.

I met my uncle who had just arrived.

In this integrated relative clause, the antecedent is "uncle"; the relative clause combines with this to form the nominal "uncle who had just arrived, and "my" indicates that this nominal provides an identifying description of the referent.

The relative clause thus provides part of the identifying description, hence the implicature that there is some other uncle from which the one who has just arrives needs to be distinguished.

In the supplementary equivalent ("I met my uncle, who had just arrived"), the antecedent is "my uncle": this is marked as definite, indicating that the description "uncle" is assumed to be sufficient in the context to identify the referent.

Edit: Genitive determiners like "my" are always definite. In the above example "my uncle" corresponds to "the uncle of mine", not "an uncle of mine".