His Excellency joining newly-weds in Scottish isles (8)
I believe the answer is:
hebrides
'scottish isles' is the definition.
(the Hebrides are a Scottish archipelago)
'his excellency joining newly-weds' is the wordplay.
'his excellency' becomes 'he'.
'joining' is a charade indicator (letters next to each other).
'newly-weds' becomes 'brides'.
'he'+'brides'='HEBRIDES'
'in' acts as a link.
(Other definitions for hebrides that I've seen before include "Island group off NW Scotland" , "His breed comes from west of Scotland" , "Island group (New, Inner and Outer)" , "overture" , "Muck out here" .)