Unlawful act beginning to annexe part of Ukraine (6)
I believe the answer is:
crimea
'part of ukraine' is the definition.
(Crimean peninsula in Ukraine)
'unlawful act beginning to annexe' is the wordplay.
'unlawful act' becomes 'crime' (I've seen this before).
'beginning to' says to take the initial letters.
The first letter of 'annexe' is 'a'.
'crime'+'a'='CRIMEA'
(Other definitions for crimea that I've seen before include "A crime committed here on the northern coast of the Black Sea?" , "Area of Ukraine" , "Scene of the charge of the Light Brigade" , "European peninsula" , "former battleground" .)