Organisation of great day for the Allies captured in an instant (8)
I believe the answer is:
movement
'organisation' is the definition.
(a political organisation)
'great day for the allies captured in an instant' is the wordplay.
'great day for the allies' becomes 'VE' (VE Day, Victory in Europe Day).
'captured in' means one lot of letters goes inside another.
'an instant' becomes 'moment' (synonyms).
've' going within 'moment' is 'MOVEMENT'.
'of' is the link.
(Other definitions for movement that I've seen before include "Part of a musical work" , "campaign" , "A gesture" , "Having cause" , "Motion" .)