Sealing sheet (6)
I believe the answer is:
gasket
'sealing sheet' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for gasket that I've seen before include "Stage K (anag.)" , "Seal at a pipe point" , "Seal for a pipe joint" , "Flat metal seal between tow surfaces as of engine" , "sandwich filling" .)