Island country sides with president, almost certain to make a comeback (6)
I believe the answer is:
cyprus
'island country' is the definition.
(Cyprus is an island)
'sides with president almost certain to make a comeback' is the wordplay.
'sides with' becomes 'cy' (I can't justify this - if you can you should give a lot more credence to this answer).
'president' becomes 'p' (common abbreviation eg in VP).
'almost' means to remove the last letter (most of the word but not all of it).
'certain' becomes 'sure' (synonyms).
'to make a comeback' says the letters should be written in reverse (I've seen 'making a comeback' mean this).
'sure' with its final letter removed is 'sur'.
'sur' in reverse letter order is 'rus'.
'cy'+'p'+'rus'='CYPRUS'
(Other definitions for cyprus that I've seen before include "Island - legendary home of Aphrodite" , "Aphrodite's island" , "Mediterranean island, capital Nicosia" , "Othello island" , "European island country" .)