Matilda, therefore, no longer put into quarantine (7)
I believe the answer is:
isolate
'put into quarantine' is the definition.
(I've seen this before)
'matilda therefore no longer' is the wordplay.
I cannot quite understand how this works, but
'therefore' could be 'so' and 'so' is found in the answer.
This explanation may well be incorrect...
(Other definitions for isolate that I've seen before include "Pick out" , "Place or set apart" , "Strand" , "Set apart, quarantine" , "Eat soil (anag)" .)