Airtight (6)
I believe the answer is:
sealed
'airtight' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sealed that I've seen before include "Secured against leakage" , "Closed (of lips, envelope, tomb etc)" , "Fastened as in S.W.A.L.K." , "Leased in secured state" , "Stamped as authentic" .)