Make still or quiet (6)
I believe the answer is:
becalm
'make still or quiet' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for becalm that I've seen before include "Make quiet or still - sounds like an order" , "Render motionless from lack of wind" , "Leave a sailing vessel unable to move for lack of wind" , "Leave (sailing ship) without wind" , "Leave (sailing ship) deprived of power" .)