Final message is being sealed in case (6)
I believe the answer is:
valise
'case' is the definition.
(valise is a kind of case)
'final message is being sealed' is the wordplay.
I cannot quite understand how this works, but
'is' is present in the answer.
The remaining letters 'vale' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for valise that I've seen before include "A small suitcase for Leavis" , "Overnight case" , "Elvis A? (anag)" , "Small overnight bag" , "Travel bag" .)