Stop - this is a hold-up (4)
I believe the answer is:
stem
'stop' is the definition.
(I know that stem is a more specific form of the action check)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for stem that I've seen before include "Shoot; arrest" , "Main central part as of plant or glass" , "'Stalk, shank (4)'" , "Dam" , "Restrain - staunch - supporter" .)