Aggressive dog runs after copper (3)
I believe the answer is:
cur
'aggressive dog' is the definition.
(I've seen this before)
'runs after copper' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'after' is a charade indicator (letters next to each other).
'copper' becomes 'cu' (chemical symbol).
'r' put after 'cu' is 'CUR'.
(Other definitions for cur that I've seen before include "Wretch" , "Inferior dog or one of mixed breed" , "Miserable dog" , "Worthless mongrel" , "Worthless or bad-tempered dog" .)