Revealed first three are missing when shut (6)
I believe the answer is:
closed
'shut' is the definition.
(I know that shut can be written as closed)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for closed that I've seen before include "Shut" , "Resolved" , "successfully completed" , "Terminated" , "clinched the sale" .)