in the saying 'this means trouble', is trouble the adjective. I know that an adjective is a describing word but not sure if this works for this saying.

Thanks.

The word "trouble" is a noun, so in this saying, it seems to be the direct object.

I see what you mean, though -- it's more of a descriptor here than a thing or idea. I guess you could say it's a noun acting like a predicate adjective in an idiomatic expression.