Cascade causing split below rocky peak (7)
I believe the answer is:
torrent
'cascade' is the definition.
(thesaurus)
'split below rocky peak' is the wordplay.
'split' becomes 'rent' ('rent' can be a synonym of 'split').
'below' means one lot of letters go next to another.
'rocky peak' becomes 'tor' (I've seen this before).
'rent' put after 'tor' is 'TORRENT'.
'causing' acts as a link.
(Other definitions for torrent that I've seen before include "Stream in spate" , "Heavy stream" , "Overwhelming deluge" , "Flood, spate" , "Fast-moving stream" .)