Some elements of answers :

About the "what is fair use": I suggest you to read this answer about fair use, especially "Factor 3 : Amount and substantiality of the portion used in relation to the copyrighted work as a whole". Even if you only take a little part of the definition but it represents the whole core, then it may not be fair use (still, I am not a lawyer nor a judge).

About the Creative Commons with NC: For my interpretation (and I insist on that point), it is the wordlist as a whole which is licenced CC-BY-NC, understand: the whole collection and the way information have been processed to make this product. So using the wordlist or a part of the wordlist without modification in a commercial way would not be allowed.

Yet, I think you're still allowed to work on the wordlist to re appropriate the content, learn and process the information to create your own wordlist. Nobody will forbid you to learn a word and its definition, then to create your own wordlist.

I don't know how are the wordlists, but they may fall into the Sui Generis Database Right : "An owner has the right to object to the copying of substantial parts of their database, even if data is extracted and reconstructed piecemeal."

"Database rights are independent of copyright: The arrangement, selection, and presentation of the data may be protected by copyright, while the database as a whole can be protected by database right."

Some elements of answers :

About the "what is fair use": I suggest you to read this answer about fair use, especially "Factor 3 : Amount and substantiality of the portion used in relation to the copyrighted work as a whole". Even if you only take a little part of the definition but it represents the whole core, then it may not be fair use (still, I am not a lawyer nor a judge).

About the Creative Commons with NC: For my interpretation (and I insist on that point), it is the wordlist as a whole which is licenced CC-BY-NC, understand: the whole collection and the way information have been processed to make this product. So using the wordlist or a part of the wordlist without modification in a commercial way would not be allowed.

Yet, I think you're still allowed to work on the wordlist to re appropriate the content, learn and process the information to create your own wordlist. Nobody will forbid you to learn a word and its definition, then to create your own wordlist.

I don't know how are the wordlists, but they may fall into the Sui Generis Database Right : "An owner has the right to object to the copying of substantial parts of their database, even if data is extracted and reconstructed piecemeal."

"Database rights are independent of copyright: The arrangement, selection, and presentation of the data may be protected by copyright, while the database as a whole can be protected by database right."