Keep most important article in metal 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 metal 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' indicates putting letters inside.
'metal container' becomes 'tin' (I've seen this in another clue).
'a' placed into 'tin' is 'tain'.
'main'+'tain'='MAINTAIN'
(Other definitions for maintain that I've seen before include "finance" , "Claim" , "Keep in safety and protect from harm and decay" , "declare" , "Keep up - carry on" .)