Alas, report 550 cracks (5)
I believe the answer is:
sadly
'alas' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sadly that I've seen before include "Unhappily" , "Lamentably, unfortunately" , "Regrettably" , "Mournfully" , "I regret to say" .)