In which we serve on the level? (7)
I believe the answer is:
uniform
'the level?' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for uniform that I've seen before include "a soldier must assume this" , "Military clothes" , "Even; clothing" , "Distinctive outfit" , "Tunic; even" .)