Dish of silver good in his keeping (6)
I believe the answer is:
haggis
'dish' is the definition.
(haggis is a kind of dish)
'silver good in his keeping' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'good' becomes 'g' (abbreviation).
'in' means one lot of letters goes inside another.
'keeping' indicates putting letters inside (some letters kept inside).
'ag' placed around 'g' is 'agg'.
'agg' going inside 'his' is 'HAGGIS'.
'of' acts as a link.
(Other definitions for haggis that I've seen before include "traditional meal" , "Scottish dish boiled in an animal's stomach" , "Scots offal dish" , "boil-in-bag meal" , "Scotish sheep dish" .)