Silver found during one excavation? Fancy! (7)
I believe the answer is:
imagine
'fancy' is the definition.
(I've seen this before)
'silver found during one excavation?' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'found during' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'excavation?' becomes 'mine' (mine is a kind of excavation).
'i'+'mine'='imine'
'ag' going inside 'imine' is 'IMAGINE'.
(Other definitions for imagine that I've seen before include "Conceive; fancy" , "Picture mentally" , "Devise" , "Conceive in the mind" , "Reckon" .)