Yes you can trademark a clothing line.
If you are using a particular word, graphic, phrase or slogan on a Clothing Line, you can trademark it to prevent any other Clothing Line manufacturers from using it without your permission. For example, while I was in Memphis I was staying at the Peabody Hotel and in the lobby is Lansky Brothers a “clothier to the King” or other more famous marks including, well take a trip to the mall to see registered trademark for a variety of Clothing Lines. A Clothing Line trademark entitles the owner of the trademark to stop others from using their mark or a similar mark on Clothing Line without their permission. Another name for Clothing Line is apparel,including jackets, coats, shirts, pants, jackets, hats, caps, uniforms, tee shirts, sweatshirts, leggings, shorts, pajamas or other specific items.
To trademark a Clothing Line, the word, graphic, phrase or slogan must be non-descriptive and unique to merit trademark protection. The United States Patent and Trademark Office will reject your trademark application if they determine that your word, graphic, phrase or slogan is too similar to a prior registered trademark.
By Mat Grell, US Trademark Attorney