Developments are not treated lightly here (8)

I believe the answer is:

darkroom

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'developments are not treated lightly here' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

This is all the clue.

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(Other definitions for darkroom that I've seen before include "Place for developing photographs" , "In which to develop photographs" , "Film-developing cubicle" , "Place to develop films" , "development area" .)

I've seen this clue in The Irish News.

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