Assume one quarry contains silver (7)
I believe the answer is:
imagine
'assume' is the definition.
(I've seen this before)
'one quarry contains silver' is the wordplay.
'one' becomes 'i' (Roman numeral).
'quarry' becomes 'mine'.
'contains' indicates putting letters inside.
'silver' becomes 'ag' ('Ag' can be a synonym of 'silver').
'mine' going around 'ag' is 'magine'.
'i'+'magine'='IMAGINE'
(Other definitions for imagine that I've seen before include "Conceive; fancy" , "Assume" , "See something that doesn't exist" , "Picture in mind, envisage" , "1971 John Lennon song" .)