Suppose it’s the southern part of Ayrshire (3)
I believe the answer is:
Here is my best explanation:
'suppose' is the definition.
'the southern part of ayrshire' is the wordplay.
'the southern' becomes 's' (abbreviation).
'part of ayrshire' becomes 'ay' (I can't justify this - if you can you should give a lot more credence to this answer).
'it's' is the link.