Stay out after Newton gets aggressive (5)
I believe the answer is:
nasty
'aggressive' is the definition.
'stay out after newton' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'after' is a charade indicator (letters next to each other).
'newton' becomes 'n' (synonyms).
'stay' is an anagram of 'asty'.
'asty' after 'n' is 'NASTY'.
'gets' acts as a link.
(Other definitions for nasty that I've seen before include "Beastly" , "Horrible" , "Horrid, repugnant" , "Mean" , "Not nice" .)