Unexpected round-up Bart arranged (6)
I believe the answer is:
abrupt
'unexpected' is the definition.
(I've seen this in another clue)
'round-up bart arranged' is the wordplay.
'round' is an insertion indicator.
'arranged' indicates anagramming the letters.
'bart' is an anagram of 'abrt'.
'up' placed within 'abrt' is 'ABRUPT'.
(Other definitions for abrupt that I've seen before include "Rough and quick" , "Short, sharp or sudden" , "Unexpected - terse" , "Sudden; hasty, curt" , "Terse and sudden" .)