What the pupils did when the lights went out (7)
I believe the answer is:
dilated
'what the pupils did when' is the definition.
I can't judge whether this defines the answer.
'lights went out' is the wordplay.
'lights' becomes 'late' (I can't justify this - if you can you should give a lot more credence to this answer).
'went' becomes 'did' (doing is a kind of going).
'out' indicates putting letters inside.
'late' placed into 'did' is 'DILATED'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for dilated that I've seen before include "like pupils in the dark" , "Made more open/wide" , "Enlarged" , "Spoke at length" , "Widened; expanded" .)