The French could quarantine sick in this city (5)
I believe the answer is:
lille
'this city' is the definition.
(I've seen this before)
'the french could quarantine sick' is the wordplay.
I cannot really understand how this works, but
'sick' could be 'ill' (synonyms) and 'ill' is found in the answer.
'the french' could be 'le' ('the' in French) and 'le' is found within the answer.
No letters remain.
This explanation may well be incorrect...
'in' acts as a link.
Can you help me to learn more?
(Other definitions for lille that I've seen before include "City in northern France, Flanders" , "Eurostar stop" , "City of northern France, close to Belgium" , "Stop before Paris" , "European city" .)