A successful conclusion rests with the coalition (6)
I believe the answer is:
allies
'the coalition' is the definition.
(allies is a kind of coalition)
'a successful conclusion rests' is the wordplay.
'conclusion' says to take the final letters.
'rests' becomes 'lies' (lying is a kind of resting**).
The final letter of 'successful' is 'l'.
'a'+'l'+'lies'='ALLIES'
'with' is the link.
(Other definitions for allies that I've seen before include "Countries working together" , "Joins forces" , "Formally cooperating countries" , "People or nations joined against an enemy" , "Nations aligned against Germany in WW2" .)