Sudden rap, but goes awry (6)
I believe the answer is:
abrupt
'sudden' is the definition.
(similar in meaning)
'rap but goes awry' is the wordplay.
'goes awry' is an anagram indicator (I've seen 'go awry' mean this (I've seen this in other clues)).
'rap'+'but'='rapbut'
'rapbut' with letters rearranged gives 'ABRUPT'.
(Other definitions for abrupt that I've seen before include "Curt, sharp" , "Snap!" , "Sudden, brusque" , "Unexpected - terse" , "Sudden; precipitous" .)