Girl originally from a Scottish island? (5)
I believe the answer is:
Here is my best explanation:
'girl' is the definition.
'originally from a scottish island?' is the subsidiary indication.
'originally' says to take the initial letters.
'a scottish island?' becomes 'iona' (I've seen this before**).
The first letter of 'from' is 'f'.
(Other definitions for fiona that I've seen before include "Girl's name, meaning fair in Gaelic", "Girl's name (not Flora)", "Girl" and "Name".)