Level of command (4)
'level of command' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for echelon that I've seen before include "Degree" , "Level in hierarchy" , "Tier" , "position in society" , "Bishops, say" .)
(Other definitions for rank that I've seen before include "Line of people" , "Not qualified to give order" , "fare might be found here" , "standing order" , "stink in > class" .)