i think it is not possible to create a segment of this use case just with out of the box functionality. this is because the segments just looks at a single unique visitor and his behavior when using any conditions. and while looking at this behavior and the variables you fetched, you can‘t say what is a top value or not.
remark: at least now when you can‘t use calculated metrics in the segment builder...
workarounds:
by now i can think of 3 ways how you can achieve your goal (or something similar)
1) create a calculated metric that shows the top search terms and use this metric within any reports
2) if that‘s not enough you can use the excel report builder to do manual modification on search terms (e.g. split longer phrases in short terms...)
3) if you really need a segment you could do a classification on your search term variable (for example just use the top search terms and give them a classification „top“ or use any advanced technique as written in 2 to get more granualy top search terms). afterwads you can create a segment with the „top search terms“ (based on the created classification).
remark: this classificaion needs to be updated fro time to time if you want the latest „top search terms...)
hope that helps... kind regards
