Some Parisian keeping chest with silver for stagnant times (4,4)
I believe the answer is:
dark ages
'stagnant times' is the definition.
(stagnant period of history)
'some parisian keeping chest with silver' is the wordplay.
'some parisian' becomes 'des' ('some' in French).
'keeping' indicates putting letters inside.
'chest' becomes 'ark' (historical term for a chest).
'with' means one lot of letters go next to another.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'ark'+'ag'='arkag'
'des' placed around 'arkag' is 'DARK AGES'.
'for' acts as a link.
(Other definitions for dark ages that I've seen before include "Period c AD500-1100" , "Period in Europe approx 500 - 1100 AD" , "Historical period between 500 and 1000" , "'Period of history between classical antiquity and the Italian Renaissance (4,4)'" , "Period after the fall of the Roman Empire" .)