"The clothes, which they have bought, are very expensive."
Translation:De kleren, die zij gekocht hebben, zijn heel duur.
'Kleding' has no plural form, and 'kleren' has no singular form, so you are correct!
They can be used interchangeably, but make sure that when either of those words is the subject of the sentence, you conjugate the verb accordingly, just like in English:
- De kleding hangt in de kast - The clothing is hanging in the closet.
- De kleren hangen in de kast - The clothes are hanging in the closet.