Stockholder is ready for business, withholding nothing (3)
I believe the answer is:
pen
'nothing' is the definition.
Although both the answer and definition are singular nouns, I don'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 pen that I've seen before include "Small enclosure" , "Animal enclosure" , "Confine the writer" , "See 20" , "Enclosure; a swan" .)