Separation anxiety disorder is the child or adolescent's excessive worry and apprehension about being away from their parents. Children with separation anxiety disorder often fear that their parents will be harmed in some way or will not return to them as promised.
fine adjective or adverb. It's an adjective functioning as a Predicative Complement. A Predicative Complement is a Complement of the verb that describes the Subject or Object. In this case it is describing the Subject. Predicative Complements can be noun phrases, adjective phrases and preposition phrases, but hardy ever - if ever at all - adverbs. The reason we use an adjective phrase here is that this phrase is describing a noun phrase. Modifiers of noun phrases are usually adjectives, not adverbs. Consider the following examples: I'm happy. I'm cold. I'm assiduous. *I'm happily. (adverb as PC, ungrammatical) *I'm coldly.
An adjective is a word or set of words that modifies (i.e., describes) a noun or pronoun. Adjectives may come before the word they modify. Examples: That is a cute puppy. She likes a high school senior. Adjectives may also follow the word they modify: Examples: That puppy looks cute. The technology is state-of-the-art.