If you search for instructions on cropping in Audacity, you’ll get a lot of people explaining how to do two things that you don’t want to do:
1. Remove a selected section (that is, cutting, the opposite of cropping)
2. Save a selection to a file
If you just want to select a range and remove everything on both sides, neither of those pieces of advice help. The confusion is caused by people answering questions they don’t understand, and the fact that Audacity uses the term “Trim” instead of crop. Of course, if you read an article like this one, you will be even more confused, because he uses the term “Trim” to mean Cut!
Anyhow, all you have to do is select the range you want, then Edit -> Remove Audio -> Trim. Or just hit Ctrl-T.
Update: Newer versions of Audacity moved Trim to Edit -> Remove Special -> Trim Audio.