One thus delayed in quarantine (7)
I believe the answer is:
isolate
'in quarantine' is the definition.
(I've seen this before)
'one thus delayed' is the wordplay.
'one' becomes 'i' (Roman numeral).
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'delayed' becomes 'late' (I've seen this before).
'i'+'so'+'late'='ISOLATE'
(Other definitions for isolate that I've seen before include "Separate and keep away from other people" , "Maroon" , "Single out" , "Put into quarantine" , "Strand" .)