Pens and pencils, they say, aren't moving! (10)
I believe the answer is:
stationary
'moving' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for stationary that I've seen before include "Motionless" , "Fixed" , "Unmoving" , "Static" , "Not moving" .)