Treacherous reactionaries excoriated civil rights efforts and needled their leaders (8)
I believe the answer is:
recreant
'treacherous' is the definition.
(synonyms)
'civil rights efforts and needled their leaders' is the wordplay.
'civil' becomes 'rec' (I can't explain this - if you can you should give a lot more credence to this answer).
'leaders' says to take the initial letters.
The initial letters of 'rights efforts and needled their' is 'reant'.
'rec'+'reant'='RECREANT'
'reactionaries excoriated' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Other definitions for recreant that I've seen before include "Craven, apostate" , "Cowardly; apostate" , "yellow" , "disloyal" .)