Case is located in valley (6)
I believe the answer is:
valise
'case' is the definition.
(valise is a kind of case)
'is located in valley' is the wordplay.
'located in' is an insertion indicator.
'valley' becomes 'vale' (synonyms).
'is' placed inside 'vale' is 'VALISE'.
(Other definitions for valise that I've seen before include "Overnight case" , "Travel bag" , "Small piece of luggage" , "Small travelling bag or suitcase" , "Elvis A? (anag)" .)