The night before armistice, finally Victory in Europe (3)
I believe the answer is:
eve
'the night before' is the definition.
(I've seen this before)
'armistice finally victory in europe' is the wordplay.
'finally' says to take the final letters.
'victory in europe' becomes 've' (abbreviation as in 'VE Day').
The last letter of 'armistice' is 'e'.
'e'+'ve'='EVE'
(Other definitions for eve that I've seen before include "Period immediately before" , "Woman of Eden" , "One from Genesis" , "Later in the day, in short" , "when it gets dark" .)