Conceive of silver being discovered in one excavation (7)
I believe the answer is:
imagine
'conceive of' is the definition.
('imagine' can be a synonym of 'conceive of')
'silver being discovered in one excavation' is the wordplay.
'silver' becomes 'ag' ('Ag' can be a synonym of 'silver').
'being discovered in' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'excavation' becomes 'mine' (mine is a kind of excavation**).
'i'+'mine'='imine'
'ag' placed into 'imine' is 'IMAGINE'.
(Other definitions for imagine that I've seen before include "Suppose; conceive" , "Assume" , "Picture in mind, envisage" , "Reckon" , "Picture to oneself" .)