One charged over a Scottish island (4)
I believe the answer is:
iona
'scottish island' is the definition.
(island of the Hebrides)
'one charged over a' is the wordplay.
'one charged' becomes 'ion' (type of charged particle).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'ion'+'a'='IONA'
(Other definitions for iona that I've seen before include "'Hebridean island, early centre of Celtic Christianity (4)'" , "Christian settlement" , "Scottish land known for St Columba" , "part of Scotland" , "Holy island" .)