Night fell over development area (8)
I believe the answer is:
darkroom
'development area' is the definition.
(I've seen this before)
'night fell over' is the wordplay.
'night' becomes 'dark' ('dark' can be a synonym of 'night').
'fell' becomes 'moor' (I've seen this before).
'over' is a reversal indicator.
'moor' reversed gives 'room'.
'dark'+'room'='DARKROOM'
(Other definitions for darkroom that I've seen before include "Film developer's workplace" , "In which to develop photographs" , "Gloomy place where photos are processed" , "Film-developing cubicle" , "Place for developing photographs" .)