Is ubrania neutral?
The singular, "ubranie," is neutral; however, this form ("moje ubrania") is actually plural.
So the possessive always agrees with the nominative case of the noun?
The possessive just has to agree with the case. Here it's just a phrase, so it's Nominative.
If it was Genitive, for example, that would be "moich ubrań".
How do you define a singular 'clothes'?
The word "ubranie" basically means one set of clothes. For example the clothes I am wearing at the moment, the clothes I put on my chair to wear tomorrow, some elegant clothes to wear on special occasions, etc.
So it's like an outfit. Ok thanks.