Handsome youth lives by a Scottish river (6)
I believe the answer is:
adonis
'handsome youth' is the definition.
(I've seen this in another clue)
'lives by a scottish river' is the wordplay.
'lives' becomes 'is' ('be' can be a synonym of 'live').
'by' means one lot of letters go next to another.
'scottish river' becomes 'Don' (River Don in Scotland).
'a'+'don'='adon'
'is' put after 'adon' is 'ADONIS'.
(Other definitions for adonis that I've seen before include "Type of male beauty" , "Aphrodite's love" , "A very handsome ancient Greek" , "lovely man" , "No aids (anag)" .)