Not all castanets are brown! (3)
I believe the answer is:
tan
'brown' is the definition.
(I know that tan is a type of light brown)
'not all castanets are' is the wordplay.
'not all' indicates the answer is hidden within the clue (only some of the letters).
'TAN' can be found hidden inside 'castanets are'.
(Other definitions for tan that I've seen before include "Yellowish-brown colour" , "Convert hide to leather" , "Turn hide into leather" , "Nice brown induced by sun" , "Bark!" .)