Leave group by Scottish river (3,5)
I believe the answer is:
set forth
'leave' is the definition.
(setting forth is a kind of leaving)
'group by scottish river' is the wordplay.
'group' becomes 'set' (set is a kind of group).
'by' means one lot of letters go next to another.
'scottish river' becomes 'forth' (I've seen this before).
'set'+'forth'='SET FORTH'
(Other definitions for set forth that I've seen before include "Proclaim" , "Describe (something) in writing or speech" , "Depart" , "display" , "Begin journey or lay out" .)