Particles of dry grass crossing a river beneath mountain (5,3)

'particles' is the definition.

'dry grass crossing a river beneath mountain' is the wordplay.
'dry grass' becomes 'hay'.
'crossing' is an insertion indicator.
'river' becomes 'r'.
'beneath' is a charade indicator (letters next to each other) (in a down clue).
'mountain' becomes 'alp' (alp is a kind of mountain**).
'a'+'r'='ar'
'hay' enclosing 'ar' is 'haray'.
'haray' after 'alp' is 'ALPHA RAY'.

'of' is the link.