Planet has a harmful influence (4)
I believe the answer is:
mars
I believe this clue is a double definition.
'planet' is the first definition.
(Mars is a planet)
'has a harmful influence' is the second definition.
(to mar is to damage or harm)
(Other definitions for mars that I've seen before include "Red planet and god of war" , "one associated with war" , "Roman war god; planet" , "Next planet out" , "Damages - planet" .)