Not sure how or where to put substring-before

I have a test project where I read a record from an excel file, and then process it (using Process method of our wsdl). The process does not expect to see any XML tags in the content, but because I am processing tweets, there are attribute tags following the actual tweet. So somewhere, I want to do something like substring-before(tweet,"<"), which should just give me back the content before the first attribute tag. Only I can't figure out where to put it so that it works, and, my guess is, that is the wrong syntax. Any help?