炸鸡 is a verb-object compound (离合词) meaning 'to fry chicken'. Like other separable verbs, the verb 炸 can be separated from the object 鸡 to insert aspect markers or classifiers, as in 炸了一只鸡. Note that 炸鸡 is also widely used as a noun meaning 'fried chicken', though that usage is listed under a different part of speech.