Reach an agreement or become calm (6)
I believe the answer is:
settle
'reach an agreement or become calm' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for settle that I've seen before include "Find a home" , "Become established, make one's home" , "Colonise with a seat" , "A sort of bench" , "Fall on to floor" .)