Can I still use Defender AV if Field Effect NGAV is operating as my primary AV?

Yes! If Microsoft Defender for Endpoint is installed on your endpoints, Defender Antivirus can run in passive mode while Field Effect MDR serves as the primary antivirus solution. If you do not have Microsoft Defender for Endpoint, Defender AV will be fully disabled if Field Effect MDR is operating as your primary AV. This does not affect the state or performance of the Field Effect NGAV.