Idriess in command of the Ionians (5)
I believe the answer is:
ionic
'of the ionians' is the definition.
The definition suggests a singular noun which matches the answer.
'idriess in command' is the wordplay.
'idriess' becomes 'ion' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in command' becomes 'ic' (military abbreviation).
'ion'+'ic'='IONIC'
Can you help me to learn more?
(Other definitions for ionic that I've seen before include "Like some Greek architecture" , "One of five classical orders of architecture" , "ancient order" , "A type of classical Greek column" , "classical Greek design" .)