Outcrop torn by a downpour (7)
I believe the answer is:
torrent
'a downpour' is the definition.
(I know that downpour can be written as torrent)
'outcrop torn by' is the wordplay.
'outcrop' becomes 'tor' (a tor is a rocky outcrop).
'torn' becomes 'rent' (rending is a kind of tearing).
'by' is a charade indicator (letters next to each other).
'tor'+'rent'='TORRENT'
(Other definitions for torrent that I've seen before include "Rush of water" , "Heavy stream" , "Overwhelming deluge" , "Outburst" , "Violently fast stream of water" .)