After uninstalling Avast, Defender is again your default AV.
When you install Avast, Defender shuts itself down:
http://screencast-o-matic.com/screenshots/u/Lh/1490224299612-77080.png
You can run it as a second opinion AV if you use the Settings option in Defender,
Make the changes indicated in the Window that opens:
http://screencast-o-matic.com/screenshots/u/Lh/1490224127948-77478.png
Once done, you can now use Defender on demand for a second opinion.