In a nasty legalistic manner (5)
I believe the answer is:
style
'manner' is the definition.
(I know that manner can be written as style)
'in a nasty legalistic' is the wordplay.
'in' says the answer is hidden in the clue.
'STYLE' is hidden in the letters of 'nasty legalistic'.
(Other definitions for style that I've seen before include "Former writer" , "Chic" , "Mode; fashionableness" , "Technique" , "With 1a [NEW] dating from 1752" .)