Sorry for decision to expel student (5)
I believe the answer is:
ruing
'sorry' is the definition.
(to rue is to regret or feel sorrow)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ruing that I've seen before include "full of remorse?" , "Bitterly regretting" , "feeling remorse" , "Feeling sorry for" , "Having second thoughts" .)