Silver found in one excavation? Fancy! (7)
I believe the answer is:
imagine
'fancy' is the definition.
(I know this)
'silver found in one excavation?' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'found in' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'excavation?' becomes 'mine' (mine is a kind of excavation).
'i'+'mine'='imine'
'ag' placed within 'imine' is 'IMAGINE'.
(Other definitions for imagine that I've seen before include "Picture in mind, envisage" , "Picture mentally" , "1971 John Lennon song" , "Picture to oneself" , "Suppose; conceive" .)