He explains to the pupils what has happened (7,6)

'pupils what has happened' is the definition.
I know nothing about this answer so I can't judge whether this works.

'he explains to the' is the wordplay.
I cannot quite see how this works, but
'he' could be 'hi' and 'hi' is located in the answer.
'to' is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?