Harriet was a clever child,

The adjective in this question

The adjective in the above statement is Clever

Explanation:

Adjective is generally used to glorify or describe a noun. It can make a sentence sound more interesting.

For example:

Mary has a collection of Russian dolls is a statement without adjective where,

Mary has a collection of expensive Russian dolls is a statement with adjective and you can clearly see that the sentence with adjective gives more detailing to that sentence.

In the above given statement, Clever adds more detail to the statement and describes the noun, Hence we can conclude Clever as an adjective.