Take over from magistrate, about 51 (7)
I believe the answer is:
relieve
'take over from' is the definition.
('relieve' can be a synonym of 'take over from')
'magistrate about 51' is the wordplay.
'magistrate' becomes 'reeve' (I've seen this before).
'about' indicates putting letters inside.
'51' becomes 'li' (roman numeral).
'reeve' placed around 'li' is 'RELIEVE'.
(Other definitions for relieve that I've seen before include "Ease pain or burden" , "Comfort" , "Assuage - take over duty from" , "Alleviate, assuage" , "Take over work from someone" .)