Beat Nathaniel up! (3)
I believe the answer is:
tan
'beat' is the definition.
(tan can mean to beat or hit)
'nathaniel up' is the wordplay.
'nathaniel' becomes 'nat' (I've seen this in other clues).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'nat' in reverse letter order is 'TAN'.
(Other definitions for tan that I've seen before include "Turn dark in sun" , "Get the effect of the sun." , "Catch the sun" , "Lie in sun" , "Nice brown induced by sun" .)