Do you mean the first component of the directory? If so, turn the path into a list of components, then extract the first component. I assume you're using GNU make, and you're not doing something insane like expect file names with whitespace to go through unscathed. Don't try this with an absolute path.

BUILD_DIR = dir/subdir/.../sub-a-dub-dir
distclean:
        rm -rf $(firstword $(subst /, ,$(BUILD_DIR)))

(This looks quite dangerous. What if someone wants to set an alternate build directory and runs make BUILD_DIR=$PWD/my/build/dir?)

Do you mean the first component of the directory? If so, turn the path into a list of components, then extract the first component. I assume you're using GNU make, and you're not doing something insane like expect file names with whitespace to go through unscathed. Don't try this with an absolute path.

override BUILD_DIR = dir/subdir/.../sub-a-dub-dir
distclean:
        rm -rf $(firstword $(subst /, ,$(BUILD_DIR)))

I make BUILD_DIR override whatever the user might have passed on the command line, to avoid someone accidentally passing it and having something unintended deleted.