Not standing for harming a steed (6)
I believe the answer is:
seated
'not standing' is the definition.
('seated' is the opposite to 'standing')
'harming a steed' is the wordplay.
'harming' is an anagram indicator (I've seen 'harmed' mean this).
'a'+'steed'='asteed'
'asteed' is an anagram of 'SEATED'.
'for' is the link.
(Other definitions for seated that I've seen before include "Given chair" , "not standing for it" , "Sedentary" , "Sitting in sedate way" .)