Flood coming from fissure under hill (7)
I believe the answer is:
torrent
'flood' is the definition.
(a torrent is a large amount of water)
'fissure under hill' is the wordplay.
'fissure' becomes 'rent' (both can mean a rip or rift).
'under' means one lot of letters go next to another.
'hill' becomes 'tor' (tor is a kind of hill).
'rent' after 'tor' is 'TORRENT'.
'coming from' acts as a link.
(Other definitions for torrent that I've seen before include "Strong stream" , "Turbulent stream of water" , "Raging flood" , "Rapid stream" , "Rush of water" .)