Quarantine is over, not before time (7)
I believe the answer is:
isolate
'quarantine' is the definition.
(I know that quarantine is a more specific form of isolate)
'is over not before time' is the wordplay.
'over' becomes 'o' (cricket abbreviation).
'not before time' becomes 'late' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'is'+'o'+'late'='ISOLATE'
Can you help me to learn more?
(Other definitions for isolate that I've seen before include "Cut off from all contact" , "Strand" , "Pick out" , "Segregate or quarantine" , "split up" .)