Unfortunate dismissal over a poor grade (3,3)
I believe the answer is:
too bad
'unfortunate' is the definition.
(similar in meaning)
'dismissal over a poor grade' is the wordplay.
'dismissal' becomes 'boot' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'over' says the letters should be written backwards.
'poor grade' becomes 'D' (an exam result).
'boot' back-to-front is 'toob'.
'toob'+'a'+'d'='TOO BAD'
Can you help me to learn more?
(Other definitions for too bad that I've seen before include "Informally regrettable, a pity" , "What a pity, hard luck" , "Tough" , "A pity, being excessively naughty" , "Informally regrettable - excessively wicked" .)