Tranquil silence (4)
I believe the answer is:
hush
'tranquil silence' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hush that I've seen before include "Call for silence" , "(Call for) quiet" , "Make or become silent or quiet" , "Noiselessness" , "Make (someone) quiet" .)