Magistrate arresting instigator of dirty trick (5)
I believe the answer is:
dodge
'trick' is the definition.
(I've seen this before)
'magistrate arresting instigator of dirty' is the wordplay.
'magistrate' becomes 'doge' (doge is a kind of magistrate).
'arresting' is an insertion indicator.
'instigator of' says to take the initial letters (the letter which begins or 'instigates' the word).
The first letter of 'dirty' is 'd'.
'doge' enclosing 'd' is 'DODGE'.
(Other definitions for dodge that I've seen before include "Western city" , "Avoid; cunning trick" , "Elude, sidestep" , "Cunning move" , "Evade cunningly" .)