make is pretty smart, and picks up what has changed from the last build, so if you run repo sync and then build without cleaning, in most cases it should work just fine.
make clean does the same thing as make clobber, which is removing all generated files ($OUT_DIR).
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer?