There's a scrap when a communist is told to be quiet (5)
I believe the answer is:
shred
'there's a scrap' is the definition.
(both can mean a small amount of material)
'a communist is told to be quiet' is the wordplay.
I cannot really understand how this works, but
'communist' could be 'red' (red can informally mean communist) and 'red' is located in the answer.
'quiet' could be 'sh' ('sh' is used to tell someone to be quiet) and 'sh' is found in the answer.
No letters remain.
This explanation may well be incorrect...
'when' is the link.
Can you help me to learn more?
(Other definitions for shred that I've seen before include "Torn piece of material" , "Tiny amount" , "Cut finely" , "Tear into scraps" , "Torn strip of cloth" .)