The Inkscape src directory already contains a .clang-format file, so all you need to do is:

$ clang-format src/live_effects/lpe-fillet-chamfer.cpp

Even if you are many levels deep in the src dir, clang-format finds the .clang-format file at the topmost dir.

Another useful option to know is:

$ clang-format -assume-filename=/home/samuel/code/lang/c++/.clang-format

in which you can put a .clang-format file anywhere you want, and you can call it whatever you want, assuming you alter the filename specified in the cmdline option accordingly.

Samuel