Arrogant thug beaten up in grass (7)
I believe the answer is:
haughty
'arrogant' is the definition.
(I've seen this in another clue)
'thug beaten up in grass' is the wordplay.
'beaten up' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'grass' becomes 'hay' (hay is dried grass).
'thug' anagrammed gives 'ught'.
'ught' going into 'hay' is 'HAUGHTY'.
(Other definitions for haughty that I've seen before include "Imperious, arrogant" , "Supercilious" , "Proud, arrogant" , "Disdainful" , "Showing arrogant superiority and disdain" .)