Bad actor by Spanish river foolishly took it from a goat (6)
I believe the answer is:
mohair
'bad' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for mohair that I've seen before include "Fabric from coating of angora goat" , "Fine goat's cloth" , "Fine hair fabric" , "Fabric made from the coat of an Angora goat" , "Angora fleece" .)