Magistrate with monarch is mug (6)
I believe the answer is:
beaker
'is mug' is the definition.
(I've seen this before)
'magistrate with monarch' is the wordplay.
'magistrate' becomes 'beak' (informal term).
'with' means one lot of letters go next to another.
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'beak'+'er'='BEAKER'
(Other definitions for beaker that I've seen before include "Wide-mouthed cup" , "Open glass, straight with lip, used in laboratory" , "Plastic mug" , "Cup with a wide mouth" , "Glass vessel" .)