。
In a word, she's a dreamer.
Literal
She [topic-は] one-word [in-で] if-say dreamer is.
一言でいえば ('in a word / to put it in a single word') is a fixed framing phrase used to summarize someone's character, opinion, or situation in a tight phrase. The で marks 一言 as the means/manner ('using one word'), and いえば is the ば-conditional of 言う ('to say') functioning as 'if I were to say.' 夢想家 ('day-dreamer, idealist, one absorbed in fantasies') is built with the productive suffix ~家 ('one specialized in / characterized by'), seen also in 政治家 (politician), 芸術家 (artist), 音楽家 (musician).