Pledge is quiet and frivolous (6)
I believe the answer is:
plight
'pledge' is the definition.
(plight is a kind of pledge)
'quiet and frivolous' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano').
'and' says to put letters next to each other.
'frivolous' becomes 'light' (I've seen this before).
'p'+'light'='PLIGHT'
'is' acts as a link.
(Other definitions for plight that I've seen before include "Unlucky situation; promise" , "pickle" , "Jam" , "Predicament, difficulty" , "Alarming predicament" .)