Tightly closed (6)
'tightly closed' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for shut that I've seen before include "Slam" , "Push to" , "See 19" , "Close" , "Closed" .)
(Other definitions for sealed that I've seen before include "Secured against leakage" , "Shut up tight" , "Fastened as in S.W.A.L.K." , "Stamped as authentic" , "Leased in secured state" .)