Say nothing when incarcerated (4,2)
I believe the answer is:
shut up
'say nothing' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for shut up that I've seen before include "Close your mouth!" , "Imprisoned" , "jail" , "Rude order to stop talking" , "Stop talking - close the shop" .)