Trick with which Venice's magistrate entertains Duke (5)
I believe the answer is:
dodge
'trick' is the definition.
(I've seen this before)
'venice's magistrate entertains duke' is the wordplay.
'venice's magistrate' becomes 'doge' (the doges were magistrates of Venice).
'entertains' is an insertion indicator.
'duke' becomes 'd' (abbreviation).
'doge' enclosing 'd' is 'DODGE'.
'with which' acts as a link.
(Other definitions for dodge that I've seen before include "Avoid by sudden move" , "Get out of" , "Evade - trick" , "Avoid; cunning trick" , "Avoid - Kansas city" .)