Might one see a jay beside this river? (3)
I believe the answer is:
dee
'this river?' is the definition.
(River Dee)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for dee that I've seen before include "River of Scotland" , "Welsh banker?" , "Balmoral's river" , "Scots river" , "John ---, Elizabethan mage" .)