Greek island dismisses Russians (8)
I believe the answer is:
cossacks
'russians' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'greek island dismisses' is the wordplay.
'greek island' becomes 'cos' (I've seen this before).
'dismisses' becomes 'sacks' ('sack' can be a synonym of 'dismiss').
'cos'+'sacks'='COSSACKS'
Can you help me to learn more?
(Other definitions for cossacks that I've seen before include "Members of an elite cavalry corps in tsarist Russia" , "Famous horsemen of S Russia" , "Russian peasants?" .)