'Of' should be correct according to http://www.ldoceonline.com/dictionary/definition
Interested in Language
In the sentence below, which preposition would sound more assertive? Meaning that the definition that would follow is the correct one.
To better understand this idiom you should check the definition for/of the word...
Matthew Wai nailed it. You can't say "...definition for the word".
You could have "definition for" in a sentence such as, "We added a dye to the sample to provide better definition for scientists...".
I'm not sure what you mean by more "assertive".
Different prepositions have different uses. It depends on the sentence and situation. In this sentence, of is the best.