Very important container contains tritium (5)
I believe the answer is:
'very important' is the definition.
'container contains tritium' is the wordplay.
'container' becomes 'vial' (vial is a kind of container**).
'contains' is an insertion indicator.
'tritium' becomes 't' (abbreviation in chemistry).
'vial' enclosing 't' is 'VITAL'.
(Other definitions for vital that I've seen before include "Are such statistics" , "Essential for life; key" , "Critical, necessary" , "Essential to life, indispensable" , "Urgently needed or absolutely necessary" .)