A bit of audible tranquillity (5)
I believe the answer is:
piece
'a bit of' is the definition.
(I've seen this before)
'audible tranquillity' is the wordplay.
'audible' indicates a 'sounds like' (homophone) clue.
'tranquillity' becomes 'peace' (peace is a kind of tranquillity).
'peace' sounds like 'PIECE'.
(Other definitions for piece that I've seen before include "Component; portion" , "Unpleasant character" , "Firearm -- collector's item" , "A part of something" , "Castle, perhaps" .)