Render less effective (6)
I believe the answer is:
impair
'render less effective' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for impair that I've seen before include "blunt" , "Cause weakening of, damage" , "Damage or injure" , "Spoil, degrade" , "Courteous man (colloq.)" .)