The second-smallest of Scotland's historic counties, bordering Fife (12)
I believe the answer is:
kinrossshire
'scotland's historic counties bordering fife' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'the second-smallest of' is the wordplay.
I cannot quite see how this works, but
'second' could be 're' (prefix meaning second in eg review, re-education) and 're' is present in the answer.
'of' could be 'o' and 'o' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for kinrossshire that I've seen is " Scots county".)