Brief commander prior to treaty (7)
I believe the answer is:
compact
'brief' is the definition.
(I've seen this before)
'commander prior to treaty' is the wordplay.
'commander' becomes 'com' (I've seen this in other clues).
'prior to' is a charade indicator (letters next to each other).
'treaty' becomes 'pact' (synonyms).
'com'+'pact'='COMPACT'
(Other definitions for compact that I've seen before include "bargain" , "Tightly-knit, agreement" , "'Condensed, portable (7)'" , "Small case for face powder" , "An official agreement" .)