Old magistrate gets on with First Lady (5)
I believe the answer is:
reeve
'old magistrate' is the definition.
(I've seen this before)
'on with first lady' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'with' means one lot of letters go next to another.
'first lady' becomes 'eve' (I've seen this before).
're'+'eve'='REEVE'
'gets' acts as a link.
(Other definitions for reeve that I've seen before include "estate supervisor" , "Former magistrate" , "Bailiff, steward" , "Comic actor" , "old-fashioned official" .)