Fleeting occasion for a container (9)
I believe the answer is:
briefcase
'for a container' is the definition.
(briefcase is a kind of container)
'fleeting occasion' is the wordplay.
'fleeting' becomes 'brief' (I've seen this in another clue).
'occasion' becomes 'case' (both can mean an instance or situation).
'brief'+'case'='BRIEFCASE'
(Other definitions for briefcase that I've seen before include "Container for papers suggests a short court hearing" , "Carrier for documents" , "One may hold papers" , "Container for carrying papers" , "Portable receptacle for documents" .)