。
He was wearing a wrinkled, worn-out suit.
Literal
He [topic-は] completely-worn-out suit [object-を] wearing-was.
くたくた is a 擬態語 (state mimetic) depicting a state of exhaustion, limpness, or being worn out — can describe a person's fatigue (くたくたに疲れた 'dead tired') or a crumpled/worn-out physical object. Here it modifies 背広 ('suit,' an older word for men's formal wear, now somewhat dated compared to スーツ). ~ていた is past progressive — 'was wearing.'