Military formation (8)
(Other definitions for regiment that I've seen before include "Force under colonel" , "Organise; army unit" , "Large army unit" , "Army group" , "a whole lot of them" .)
(Other definitions for brigade that I've seen before include "Soldiers" , "Set of troops" , "Company" , "Abridge (anag)" , "Army subdivision" .)
'military formation' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for echelon that I've seen before include "Rank - status" , "Military formation; level of society" , "Body of troops arranged in a line" , "Level of command" , "series of steps" .)