the sentence: Die Kleidung ist gut.
why is it used 3rd person verb for Die Kleidung which means clothes?
Die Kleidung ist gut translates to The clothing is good. Some German words are singular but plural in English. Eg. the word for "eyeglasses" is plural in English but singular in German. "Wo ist meine Brille?" means "Where are my eyeglasses?".
Consider the other persons - first person is "I" which cannot be clothes. Second person is "you" which is also wrong. Which leaves third person singular which can stand for "it" and all singular nouns. If the noun had been plural a third person plural verb would be used. (Die Bäume sind schön). Hope that helps a little.