Rain on outbuilding — dirty film seen after this? (9)
I believe the answer is:
watershed
'dirty film seen after this?' is the definition.
(as in the TV watershed)
'rain on outbuilding' is the wordplay.
'rain' becomes 'water' (rainwater).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'outbuilding' becomes 'shed' (shed is a kind of outbuilding).
'water'+'shed'='WATERSHED'
(Other definitions for watershed that I've seen before include "Turning-point in course of events, like ridge of high land" , "Dividing ridge between two rivers" , "Turning-point in course of events, or catchment area" , "Line of high land from which streams flow in opposite directions" , "dividing line" .)