Brigade in lone arrangement with soldier (6)
I believe the answer is:
legion
'brigade' is the definition.
(thesaurus)
'lone arrangement with soldier' is the wordplay.
'arrangement' indicates anagramming the letters.
'with' indicates putting letters inside.
'soldier' becomes 'gi' (government issue or general issue).
'lone' with letters rearranged gives 'leon'.
'leon' enclosing 'gi' is 'LEGION'.
'in' is the link.
(Other definitions for legion that I've seen before include "Host" , "Many soldiers" , "Roman military formation" , "The French have a Foreign one" , "Roman army grouping" .)