International organisation thus filling in agreement (6)
I believe the answer is:
unison
'agreement' is the definition.
(I know that unison is a type of agreement)
'international organisation thus filling in' is the wordplay.
'international organisation' becomes 'UN' (United Nations).
'thus' becomes 'so' (synonyms).
'filling' indicates putting letters inside.
'so' going inside 'in' is 'ison'.
'un'+'ison'='UNISON'
(Other definitions for unison that I've seen before include "All singing the same note" , "All singing the same notes" , "Harmony - public services' trade union" , "A coincidence of musical notes" , "concert" .)