A river and, on the side, a region (3)
I believe the answer is:
tay
'a river' is the definition.
(Scottish river)
'on the side a region' is the wordplay.
'on' is a charade indicator (letters next to each other).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'side a region' becomes 'ay' (I can't justify this - if you can you should give a lot more credence to this answer).
't' next to 'ay' is 'TAY'.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for tay that I've seen before include "Longest river of Scotland" , "Scottish loch" , "Bridge at Dundee" , "Scottish river and firth" , "The - Bridge Disaster (McGonagall)" .)