For thirty seconds one has quiet at least (7)
I believe the answer is:
minimum
'least' is the definition.
(similar in meaning)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for minimum that I've seen before include "The least of smallest amount possible" , "Lower limit" , "Least recorded value" , "Nadir" , "Lowest point" .)