One's seen in valley in this case (6)
I believe the answer is:
valise
'this case' is the definition.
(valise is a kind of case)
'one's seen in valley' is the wordplay.
'one's' becomes 'is' (roman numeral).
'seen in' is an insertion indicator.
'valley' becomes 'vale' (synonyms).
'is' put into 'vale' is 'VALISE'.
'in' is the link.
(Other definitions for valise that I've seen before include "Overnight case" , "Elvis A? (anag)" , "A small suitcase for Leavis" , "Small travelling bag or suitcase" , "Travel bag" .)