Leave, determined to go on Scottish river (3,5)
I believe the answer is:
set forth
'leave' is the definition.
(setting forth is a kind of leaving)
'determined to go on scottish river' is the wordplay.
'determined' becomes 'set' ('set' can be a synonym of 'determined').
'to go on' says to put letters next to each other (in a down clue, some letters go on top of others).
'scottish river' becomes 'forth' (I've seen this before).
'set'+'forth'='SET FORTH'
(Other definitions for set forth that I've seen before include "Depart" , "Embark" , "Exhibit" , "Proclaim" , "Begin journey or lay out" .)