Military grouping (8)
'military grouping' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for division that I've seen before include "Section of organisation" , "Difference of opinion" , "Mathematical act for a body of troops." , "Formal vote in Parliament" , "Arithmetic operation" .)
(Other definitions for unit that I've seen before include "Module" , "Whole - group" , "Part of a kit" , "A single self-contained item" , "Mexican maize pancake" .)