Substitute magistrate's covering Long Island (7)
I believe the answer is:
relieve
'substitute' is the definition.
(thesaurus)
'magistrate's covering long island' is the wordplay.
'magistrate' becomes 'reeve' (I've seen this before).
'covering' means one lot of letters goes inside another.
'long' becomes 'l'.
'island' becomes 'i'.
'l'+'i'='li'
'reeve' going around 'li' is 'RELIEVE'.
(Other definitions for relieve that I've seen before include "Break the sameness of" , "Alleviate (eg, pain)" , "Take a job over from - bring aid" , "raise a siege" , "Soothe" .)