Hill split in flood (7)
I believe the answer is:
torrent
'flood' is the definition.
(thesaurus)
'hill split' is the wordplay.
'hill' becomes 'tor' (tor is a kind of hill).
'split' becomes 'rent' ('rent' can be a synonym of 'split').
'tor'+'rent'='TORRENT'
'in' is the link.
(Other definitions for torrent that I've seen before include "Turbulent flow" , "fast-moving flow" , "Turbulent, swift-flowing stream" , "Flood, spate" , "Overwhelming flood" .)