Those leading all new talks intimidate opponent (4)
I believe the answer is:
anti
'opponent' is the definition.
(someone opposed to something)
'those leading all new talks intimidate' is the wordplay.
'those leading' indicates taking the first letters (the letters which lead the words).
The initial letters of 'all new talks intimidate' is 'ANTI'.
(Other definitions for anti that I've seen before include "Adversary" , "Definitely not pro" , "One who is against (something)" , "'No' voter" , "Policy's opponent" .)