Brainiac finally shook loose most oncoming riddles in key instant (7,5,6)

'brainiac finally shook loose most oncoming riddles' is the definition.
I can't tell whether this defines the answer.

'in key instant' is the wordplay.
I cannot quite understand how this works, but
'in' is within the answer.
'key' could be 'g' (musical key) and 'g' is found within the answer.
'instant' could be 'moment' (synonyms) and 'moment' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?