Was für eins möchtest du denn? (referring back to "das Rock")
Could someone please tell me what part of speech is "eins" in the above question. I want to find some material online to explain it, but I am not sure what to "google". Is this an indefinite pronoun? It was from a shopping dialog and the person was looking for a "Kleid". Thanks.

Question

Was für eins möchtest du denn?  (referring back to "das Rock")
Could someone please tell me what part of speech is "eins" in the above question.  I want to find some material online to explain it, but I am not sure what to "google".  Is this an indefinite pronoun?  It was from a shopping dialog and the person was looking for a "Kleid".  Thanks.

Answer

I'd call this "eines" an indefinite pronoun, as it is a replacement for a noun. You can say: 
"Was für ein Kleid?" - "So ein Kleid."
"Was für eines?" - "So eines."

Mind that the word "Rock" is masculine (der Rock), and the pronoun has to change according to the grammatical gender. So with "Rock", it would be:
"Was für ein Rock?" - "So ein Rock."
"Was für einer?" - "So einer."

Answer

These pages consider "was für ein" an interrogative pronoun:
– https://deutsch.lingolia.com/de/grammatik/pronomen/interrogativpronomen
– http://donnerwetter.kielikeskus.helsinki.fi/DVG/Pronomen/Pronomen-Regeln/InterrogPron/deutsch.htm
– https://www.deutsch-als-fremdsprache-lernen.de/die-deutsche-Grammatik-lernen-die-deutschen-interrogativpronomen/
– http://canoo.net/services/OnlineGrammar/InflectionRules/FRegeln-P/Pron-was_fuer.html?MenuId=Word4253