Scotland’s longest river (3)
I believe the answer is:
tay
'scotland's longest river' 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 "Bridge at Dundee" , "Longest Scottish river" , "Dundee's river" , "Scotland's longest river" , "Scottish river and firth" .)