Embellishment in bar once thrown in fireplace (5,4)
I believe the answer is:
grace note
'embellishment in bar' is the definition.
(musical note which embellishes a bar of music)
'once thrown in fireplace' is the wordplay.
'thrown' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'fireplace' becomes 'grate' (I've seen this before).
'once' anagrammed gives 'ceno'.
'ceno' inserted into 'grate' is 'GRACE NOTE'.
(Other definitions for grace note that I've seen before include "Perhaps Nimrod's embellishment" , "musical embellishment" .)