Thank you, and now that I understand how to do it & have played around with the code, I can also confirm that combining this into a single line, that avoids the need for a new parameter is possible:

Dataset[a][Select[#in == Dataset[a][Min, "in"] &], "out"]

However, both ways the output is actually {1} rather than 1

As a quick fudge to get a value out rather than a single membered list I am using

Min[Dataset[a][Select[#in == Dataset[a][Min, "in"] &], "out"]]

create a dataset from a

b = Dataset[a]

Build a query for the value of "in"

b[Select[#in == 0 &], "out"]