The Clue
It controls the amount of light admitted (4 letter answer)
The Answer
IRIS
Tags
Notes
The answer IRIS is very common and appears about once every 48 puzzles.
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Definition
IRIS as a noun:
1. (iris, flag, fleur-de-lis, sword lily) = plants with sword-shaped leaves and erect stalks bearing bright-colored flowers composed of three petals and three drooping sepals
2. muscular diaphragm that controls the size of the pupil; it forms the colored portion of the eye
3. (iris, iris diaphragm) = diaphragm consisting of thin overlapping plates that can be adjusted to change the diameter of a central opening