Refrain from organ piece (4)
I believe the answer is:
stop
This clue is a double definition.
'refrain from' is the first definition.
(to refrain from doing something is to stop it)
'organ piece' is the second definition.
(I've seen this in another clue)
(Other definitions for stop that I've seen before include "Pack it in - bar" , "cut out" , "Red light signal" , "Discontinue (an action)" , "Standstill" .)