A clothier was a merchant who created, altered, or repaired clothing. Clothiers who specialized in dressmaking were known as dressmakers, while clothiers who specialized in men's suits and trousers were known as tailors.
In 2269, upon recognizing Vanna as the woman who had attacked him and Spock on the surface of Ardana, now clad in a gown instead of Troglyte mining wear, James T. Kirk dryly noted that she'd changed dressmakers. (TOS: "The Cloud Minders")
Elim Garak was a tailor on Deep Space 9 following its takeover by the Bajorans and the Federation. He ran a shop on the Promenade named Garak's Clothiers. He claimed to greatly dislike being a tailor, especially when compared to his past work with the Obsidian Order, but he nevertheless was extremely skilled. (DS9: "Past Prologue", "Improbable Cause", et al)
When Lieutenant Tuvok replicated Vidiian clothing for the surgically altered Commander Chakotay for his undercover away mission to Avery III, Chakotay remarked that the clothing looked like a fine piece of work. He also said that the next time he needed a tailor he would know where to look. (VOY: "Faces")