Smashing jar, by the sound of it (5)
I believe the answer is:
great
'smashing' is the definition.
(I know that smashing can be written as great)
'jar by the sound of it' is the wordplay.
'jar' becomes 'grate' (make unpleasant sound).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'grate' is a homophone of 'GREAT'.
(Other definitions for great that I've seen before include "Very good, excellent" , "Big, large" , "Enormous" , "The Plough" , "Marvellous; big" .)