Bit of tranquillity we can hear (5)
I believe the answer is:
piece
'bit of' is the definition.
(I've seen this before)
'tranquillity we can hear' is the wordplay.
'tranquillity' becomes 'peace' (peace is a kind of tranquillity).
'we can hear' shows a homophone (sound like).
'peace' sounds like 'PIECE'.
(Other definitions for piece that I've seen before include "Part - portion" , "Separate part of a whole" , "Slice" , "One part of a whole" , "Queen on board, perhaps" .)