Some clothing items like the Leather Duster have a mod slot in addition to the cosmetic slot, but I can't find any mods that will work with them.
It's been a couple of Versions since I played, but as I recall, there are two mods that work on clothing, the Storage Pocket mod (not to be confused with the Storage Pocket mod which only works on armour...) and Cargo Pocket mod which Decrease encumbrance penalty by 1 and 2 respectively.
I don't recall ever finding any, I learned how to make them from the book "Needle and Thread vol 7" and crafted them myself.
According to the wiki entry on mods, mods can only be used on Armor, specifically Light Armor or Heavy Armor, and not items that are classified as Clothing or Apparel.
The Leather Duster specifically is grouped under Clothing and Apparel, and therefore can't have any Armor mods applied to it, despite having a Modifier slot.