Terribly great intricacy about opening of negotiations? It's something guaranteed (6,9)
I believe the answer is:
racing certainty
'it's something guaranteed' is the definition.
(a racing certainty is guaranteed to happen)
'terribly great intricacy about opening of negotiations?' is the wordplay.
'terribly' indicates an anagram.
'about' is an insertion indicator.
'opening of' suggests taking the first letters.
The initial letter of 'negotiations' is 'n'.
'great'+'intricacy'='greatintricacy'
'greatintricacy' is an anagram of 'racigcertainty'.
'racigcertainty' enclosing 'n' is 'RACING CERTAINTY'.
(Other definitions for racing certainty that I've seen before include "It's never doubted" , "banker?" .)