Follow https://wiki.debian.org/SimpleBackportCreation for https://packages.debian.org/experimental/tinc (which is currently 1.1pre17)
sudo apt-get install packaging-dev debian-keyring devscripts equivs
Don't add experimental to your sources.list, its gpg keys are not in raspbian. Instead, get the source package (.dsc and tinc source code) manually: On https://packages.debian.org/experimental/tinc look for the "source package" link in the box on the right side; it is currently labeled [tinc_1.1~pre17-1.1.dsc]. Then use that link to get the .dsc and the source code, and continue with the debian backport guide:
dget http://deb.debian.org/debian/pool/main/t/tinc/tinc_1.1~pre17-1.1.dsc
cd tinc_1.1*/
sudo mk-build-deps --install --remove
dch --bpo # change version/release and add changelog entry. needed for collision avoidance
fakeroot debian/rules binary
dpkg-buildpackage -us -uc
cd ..
sudo apt install tinc_1.1*.deb
If it complains error: aborting due to unexpected upstream changes
, just make a local commit to bypass the diff check:
dpkg-source --commit