All in quarantine -- very onerous (8)
I believe the answer is:
everyone
'all' is the definition.
(I've seen this before)
'in quarantine very onerous' is the wordplay.
'in' indicates a hidden word.
'EVERYONE' is hidden within 'quarantine very onerous'.
(Other definitions for everyone that I've seen before include "Uncle Tom Cobleigh and all" , "All without exception" , "All and sundry" , "All the people" , "the masses" .)