I recently had this issue on ClearPass 6.7.x, but with a twist. My NAS is a Dell 5548P.
This solution was what I needed, but when I enabled the Avenda dictionary and set the attribute to '0', I never saw the attribute sent back in access tracker adn authentication still didn't work.
Upon exporting the Enforcement Policy, I found the following line:
<Attribute displayValue="0" value="" name="Avenda-Tag-Id" type="Radius:Avenda"/>
Replacing it with the following and re-importing the enforcemnt policy XML fixed my issue--the attribute was sent along and authentications started working.
<Attribute displayValue="0" value="0" name="Avenda-Tag-Id" type="Radius:Avenda"/>