Persian king taking power from Mediterranean island (5)
I believe the answer is:
cyrus
'persian king' is the definition.
(I've seen this in another clue)
'power from mediterranean island' is the wordplay.
'power' becomes 'p' (symbol in physics).
'from' suggests deleting specific letters.
'mediterranean island' becomes 'cyprus' (I've seen this before).
'cyprus' with 'p' taken away is 'CYRUS'.
'taking' acts as a link.
(Other definitions for cyrus that I've seen before include "American chap?" , "Great Persian king" , "Early Persian ruler" , "Ancient king of Persia" .)