Mount Etna is on ground beside river (9)
I believe the answer is:
rosinante
'mount' is the definition.
(Rosinante is Don Quixote's horse)
'etna is on ground beside river' is the wordplay.
'ground' indicates anagramming the letters ('grind' the letters into a new form).
'beside' is a charade indicator (letters next to each other) (I've seen this in other clues).
'river' becomes 'r' (abbreviation for river).
'etna'+'is'+'on'='etnaison'
'etnaison' is an anagram of 'osinante'.
'osinante' after 'r' is 'ROSINANTE'.
(Other definitions for rosinante that I've seen before include "Pathetic nag" , "Don Quixote's horse" .)