In a white area, it stains brown (3)
I believe the answer is:
tea
'stains brown' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'in a white area it' is the wordplay.
'in' indicates a hidden word.
'TEA' can be found hidden inside 'white area it'.
Can you help me to learn more?
(Other definitions for tea that I've seen before include "Light afternoon meal" , "Afternoon meal; drink" , "tips for the cup?" , "Cuppa" , "Beverage; meal" .)