Unlawful act beginning to annex part of Ukraine (6)
I believe the answer is:
crimea
'part of ukraine' is the definition.
('Crimea' is part of 'ukraine')
'unlawful act beginning to annex' is the wordplay.
'unlawful act' becomes 'crime' (I've seen this before**).
'beginning to' indicates taking the first letters.
The initial letter of 'annex' is 'a'.
'crime'+'a'='CRIMEA'
(Other definitions for crimea that I've seen before include "Black Sea region" , "Ukrainian peninsula, theatre of 1853-6 war" , "1853-56 war zone" , "War site 1853-56" , "European peninsula" .)