Suppose it’s the southern part of Ayrshire (3)
I believe the answer is:
say
'suppose' is the definition.
(synonyms)
'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).
's'+'ay'='SAY'
'it's' is the link.
Can you help me to learn more?
(Other definitions for say that I've seen before include "Opportunity to influence < > state" , "Utter words" , "Speak, declare" , "Let us suppose" , "Utter; e.g." .)