Waxes lyrical about silver sacks (7)
I believe the answer is:
ravages
'sacks' is the definition.
(I've seen this before)
'waxes lyrical about silver' is the wordplay.
'waxes lyrical' becomes 'raves' (to wax lyrical is to rave or enthuse).
'about' means one lot of letters goes inside another.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'raves' placed around 'ag' is 'RAVAGES'.
(Other definitions for ravages that I've seen before include "Causes great damage" , "a very bad series of effects" , "Sacks" .)