答案：a piece of clothing正确；a piece of clothes错误。
解析：clothes无单数形式；a piece of clothing是可以使用的。
参考资料：
引文：
WHICH WORD?
clothes / clothing
You use clothes [pl]
to talk about the things that you wear: I'll just put on
some clean clothes. You use clothing [U] to talk about clothes in general
or a particular type of clothes:
the clothing industry ◆ protective
clothing.
Note that there is no singular form of clothes. You can,
however, talk about a piece/an item/an article of clothing. Note also the
expression: I haven't got anything to wear (= any suitable
clothes) for the party tonight.