Stay drunk after knight becomes aggressive (5)
I believe the answer is:
nasty
'aggressive' is the definition.
'stay drunk after knight' is the wordplay.
'drunk' indicates an anagram.
'after' says to put letters next to each other.
'knight' becomes 'n' (chess abbreviation).
'stay' with letters rearranged gives 'asty'.
'asty' after 'n' is 'NASTY'.
'becomes' is the link.
(Other definitions for nasty that I've seen before include "rank" , "Obscene" , "grave" , "Not nice" , "Vile" .)