Illegal activity on a Black Sea peninsula (6)
I believe the answer is:
crimea
'black sea peninsula' is the definition.
(I've seen this before)
'illegal activity on a' is the wordplay.
'illegal activity' becomes 'crime' (I've seen this in another clue).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'crime'+'a'='CRIMEA'
(Other definitions for crimea that I've seen before include "Black Sea region" , "Peninsula in Ukraine where the Light Brigade charged" , "old war zone" , "historic war zone" , "A crime committed here on the northern coast of the Black Sea?" .)