One working on a Scottish island (4)
I believe the answer is:
iona
'scottish island' is the definition.
(island of the Hebrides)
'one working on a' is the wordplay.
'one' becomes 'i' (Roman numeral).
'working' becomes 'on' (on can mean working or activated).
'on' is a charade indicator (letters next to each other).
'i'+'on'+'a'='IONA'
(Other definitions for iona that I've seen before include "Hebridean location" , "Scottish land known for St Columba" , "small island" , "Island west of Mull" , "Saint Columba's isle" .)