I actually dealt with Meraki support yesterday over this issue. They had me run this command at a DOS prompt (it's the command that the agent used to check complaince)
WMIC /Namespace:\\root\SecurityCenter2 Path AntiSpywareProduct Get * /Format:List
That should listdisplayName=Windows Defender and productState=397568 (product state might vary... would have to confirm with Meraki)
If it doesn't list Windows Defender open a ticket with them... but if it does like mine did, they had me turn off the Defender on the system in question and turn it back on. What I did was disable all part of defender, waited a few minutes and rebooted. The reboot turns the Real Time Protection back on by default, and I manually turned on the other components. My system showed up secure and compliant within an hour!!