。
She speaks in such a clear tone that she's easy to understand.
Literal
She [topic-は] clear [adjectival-とした] tone [with-で] speaks [because-ので], easy-to-understand.
はっきりとした is the past-attributive form of はっきりとする — built from the mimetic-adverb はっきり ('clearly') plus と plus する, a productive way to turn mimetics and other adverbs into adjectival-state predicates. Other examples: しっかりとした ('firm, solid'), きちんとした ('proper, tidy'), ゆっくりとした ('relaxed, leisurely'). 口調 ('tone of voice, manner of speaking') describes the prosodic and stylistic dimension of speech. ~やすい ('easy to do') is the adjective-forming auxiliary that pairs with verb stems to mean 'easy to V.'