Longest river in Scotland (3)
I believe the answer is:
tay
'longest river in scotland' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tay that I've seen before include "Longest Scottish river" , "Wicklow lake" , "Scottish loch" , "Scottish river in Katayama?" , "Dundee's river" .)