Brown in an instant (3)
I believe the answer is:
tan
'brown' is the definition.
(I know that tan is a type of light brown)
'in an instant' is the wordplay.
'in' indicates a hidden word.
'TAN' is hidden in the letters of 'instant'.
(Other definitions for tan that I've seen before include "Curry" , "Thrash" , "Beat the colour" , "Convert hide to leather" , "Brown to beat" .)