What is meant by the word "مُّبِينٌ" in reference to Arabic?

In Qur'an 16:103 and 26:195, the word مُّبِينٌ (often translated 'clear' or 'pure') is used to refer to the Arabic language of the Qur'an. In linguistic terms, describing language as 'clear' means that it is easy to understand or uncomplicated. Is this the meaning of مُّبِينٌ as applied to the language of the Qur'an? Two areas of Quranic wording appear less than مُّبِينٌ. Firstly, the Quran contains muqatta'at which no one understands. Secondly, there is the concept of mutashabihat ayat, which are defined by Alsunna.org as >ayat that can have many meanings according to the rules of the Arabic language. Assigning meanings to these ayat requires thorough thinking so that acceptable meanings are given to them. Does the fact that such ayat require "thorough thinking" mean that they are still مُّبِينٌ?