Hartford continues to lose its industrial base, an economic catastrophe in the making characterized by an income base primarily dependent on companies engaged in paper shuffling.
something is in the making is a quite frequently used idiom : if someone is an athlete, star, writer, etc. in the making, they are likely to develop into that thing This young swimmer is an athlete in the making. (Cambridge dic) "an economic catastrophe in the making characterized by an income base primarily dependent on companies engaged in paper shuffling" is not a clause but a noun phrase that is in apposition to industrial base.
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catastrophe in the making = catastrophe which is in the process of being produced/generated
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in the making
In the process of developing or growing, being made, as in The editor believed this election signified history in the making. This term is frequently used to describe th