Quarantine on island long overdue (7)
I believe the answer is:
isolate
'quarantine' is the definition.
(I've seen this before)
'on island long overdue' is the wordplay.
'on' says to put letters next to each other.
'island' becomes 'is'.
'long' becomes 'o' (I can't explain this - if you can you should give a lot more credence to this answer).
'overdue' becomes 'late' (similar in meaning).
'is' put next to 'o' is 'iso'.
'iso'+'late'='ISOLATE'
(Other definitions for isolate that I've seen before include "Separate and keep away from other people" , "Cut off from all contact" , "put away?" , "Point out" , "Pick out" .)