It has to be either:
1. "Ich werde dir später antworten."
or
2. "Ich werde dir die Frage später beantworten."
You can use "beantworten" only if somebody asked you a question (which you want to answer later on - "später"). The first sentence just means you will reply to somebody (maybe a message or something like that), no matter if it is a question or not.