Keep most important article in container (8)
I believe the answer is:
maintain
'keep' is the definition.
(I know that keep can be written as maintain)
'most important article in container' is the wordplay.
'most important' becomes 'main' (main can mean the most important or major).
'article' becomes 'a' (the indefinite article in English).
'in' means one lot of letters goes inside another.
'container' becomes 'tin' (tin is a kind of container).
'a' inserted inside 'tin' is 'tain'.
'main'+'tain'='MAINTAIN'
(Other definitions for maintain that I've seen before include "declare" , "Keep in a good state of repair" , "Conserve" , "Continue with an opinion" , "Look after the upkeep" .)