Carelessly straighten, breaking to small shards (10)
I believe the answer is:
shattering
'breaking to small shards' is the definition.
'carelessly straighten' is the wordplay.
'carelessly' indicates anagramming the letters.
'straighten' with letters rearranged gives 'SHATTERING'.
(Other definitions for shattering that I've seen before include "with devastating result" , "Totally exhausting; breaking" , "very upsetting" , "being destructive" , "Very shocking" .)