。
She's a pretty clever girl.
Literal
She [topic-は] quite clever girl is.
なかなか in affirmative sentences functions as 'quite, rather, pretty (so)' — a moderate intensifier with a faint note of pleasant surprise. The same word in negative sentences shifts dramatically to mean 'not easily, doesn't readily X' (なかなか来ない, 'won't come / is slow to come'). This affirmative-vs-negative split is a textbook trap for learners. 賢い ('clever, wise, smart') leans more toward judgment-of-thinking-skill than 頭がいい (which is more about quick-wittedness or test-cleverness).