You would treat the ports clang as an alternative compiler, just like when GCC was the default.
As per the FreeBSD wiki, add the following lines to /etc/make.conf (if you want to use clang for everything, even ports), or /etc/src.conf (if you want to use clang just for world and kernel):
CC=/path/to/clang
CXX=/path/to/clang++
CPP=/path/to/clang-cpp
Check where the ports clang binaries where installed under /usr/local and adjust accordingly.