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' indicates putting letters inside.
'arranged' indicates anagramming the letters (letters rearranged).
'bart' anagrammed gives 'abrt'.
'up' inserted into '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" .)