Boy is oddly rueful on Scottish mountain (6)
I believe the answer is:
reuben
'boy' is the definition.
'oddly rueful on scottish mountain' is the wordplay.
'oddly' indicates alternate letters.
'on' is a charade indicator (letters next to each other).
'scottish mountain' becomes 'ben' (I've seen this before).
The alternate letters of 'rueful' are 'reu'.
'reu'+'ben'='REUBEN'
'is' acts as a link.
(Other definitions for reuben that I've seen before include "First child of Jacob" , "Jacob's eldest son" , "First son of Jacob (OT)" , "salt beef sandwich" , "one found in the first 6 [BOOK] of the 26 [BIBLE]" .)