The Berlin havoc ruined an offer of reconciliation (5,6)
I believe the answer is:
olive branch
'an offer of reconciliation' is the definition.
(I've seen this before)
'the berlin havoc ruined' is the wordplay.
'ruined' indicates an anagram.
'berlin'+'havoc'='berlinhavoc'
'berlinhavoc' anagrammed gives 'OLIVE BRANCH'.
(Other definitions for olive branch that I've seen before include "Emblem of peace" , "Offer of reconciliation" , "Peace gesture" , "Child reading prayer book" , "Peace offering" .)