The brass hat is not particular (7)
I believe the answer is:
general
'not particular' is the definition.
(I've seen this in another clue)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for general that I've seen before include "Rank; all-inclusive" , "High ranking officer" , "top brass" , "Rank; normal, usual" , "Enlarge for senior army officer" .)