Relieve tension (5,3,3)
I believe the answer is:
clear the air
'relieve tension' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for clear the air that I've seen before include "Dispel controversy or tension" , "way to reduce pollution?" , "To simplify the condition" , "Lower the temperature" , "Dispel differences" .)