Tidy as the admin room at the barracks (7)
I believe the answer is:
orderly
'tidy' is the definition.
(similar in meaning)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for orderly that I've seen before include "Hospital or military attendant" , "Businesslike" , "Hospital attendant; well behaved" , "Well-behaved -- soldier attending to a superior officer" , "Neat, tidy; soldier" .)