An agreement, to be brief (7)
I believe the answer is:
compact
This clue is a double definition.
'an agreement' is the first definition.
(compact is a kind of agreement)
'be brief' is the second definition.
(I've seen this before)
'to' is the link.
(Other definitions for compact that I've seen before include "Tightly-knit, agreement" , "Close - agreement - make-up case" , "Tightly grouped" , "Dense; treaty" , "Dense; agreement" .)