Persian king holding power in island (6)
I believe the answer is:
cyprus
'island' is the definition.
(Cyprus is an island)
'persian king holding power' is the wordplay.
'persian king' becomes 'cyrus' (I've seen this in another clue).
'holding' means one lot of letters goes inside another.
'power' becomes 'p' (symbol in physics).
'cyrus' placed around 'p' is 'CYPRUS'.
'in' acts as a link.
(Other definitions for cyprus that I've seen before include "Island - legendary home of Aphrodite" , "Mediterranean island of Aphrodite" , "Mediterranean island country" , "Divided Med. island" , "Partitioned Med. island" .)