。
She paid the dressmaker for the clothes.
Literal
She [topic-は] dressmaker [to-に] clothing-fee [object-を] paid.
ドレスメーカー lives more comfortably in Japanese than in modern English — 'dressmaker' is a niche word in English nowadays, but in Japanese it stays current as the standard term. に here marks the recipient of the payment. 洋服代 stacks 洋服 (Western-style clothes) with 代, a productive suffix for 'fee/charge' you'll see again in 電気代 (electricity bill), タクシー代 (taxi fare), 飲み代 (drinking-money). 支払う is the formal cousin of 払う, more at home in business and writing.