History involving military's original retreat (9)
I believe the answer is:
hermitage
'retreat' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for hermitage that I've seen before include "Recluse's abode" , "Museum housing collection started by Catherine the Great" , "accommodation for one" , "St Petersburg museum - Rhone wine" , "Place of retreat" .)