Attack magistrate without hesitation (4)
I believe the answer is:
jump
'attack' is the definition.
(jumping is a kind of attacking)
'magistrate without hesitation' is the wordplay.
'magistrate' becomes 'jp'.
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'hesitation' becomes 'um'.
'jp' enclosing 'um' is 'JUMP'.
(Other definitions for jump that I've seen before include "React with surprise" , "Spring" , "Attack" , "Sudden upward movement" , "Leap, bound" .)