If the speaker were thinking of purchasing only one single item, they would probably name that specific item instead of saying "some (new) clothes"; they might, for example, say "I need a new skirt for the summer".
With the given sentence, it's most likely the speaker has in mind purchasing multiple items in order to have either a complete outfit, or a variety of choices: they might buy 3 or 4 skirts, or a pair of pants, and a shirt or two, and some sandals; all of which would be probably lighter in both weight and color than "normal" clothes, in order to be appropriate for summer.
So a skirt definitely falls into the "clothes" category, but I wouldn't think of just one skirt as "some clothes" in that situation.