。
She was not a little surprised by the news.
Literal
She [topic-は] that news [at-に] not-a-little was-surprised.
少なからず ('not a little, considerably') is a slightly formal litotes — the negative ('not a little') functions as understatement for 'considerably,' a mild rhetorical hedge. Pairs naturally with emotion verbs: 少なからず驚く ('be considerably surprised'), 少なからず影響する ('have considerable influence'). The ~からず ending preserves the classical negative ~ず. に marks the trigger of the surprise (the news), the standard pattern for emotion-trigger に. The whole construction has a measured, written quality.