From what I understand, this can approximately be translated as:
She was wearing clothes so thin that you could say: She will definitely catch a cold.
The thing I'm having trouble with is this part in the relative clause:
First, why is 当然 at the end here? Since I don't think it is modified by what comes before, is this order used to avoid 当然、風邪を引いて which may create a confusion with "She was of course wearing blabla..."? And secondly, why this て form? Is it just an unfinished sentence implying something else, or is it to avoid 風邪を引く当然 were 風邪を引く would act as a relative clause for 当然?