Fancy one finding silver in quarry (7)
I believe the answer is:
imagine
'fancy' is the definition.
(I've seen this before)
'one finding silver in quarry' is the wordplay.
'one' becomes 'I' (Roman numeral).
'finding' says to put letters next to each other.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'in' is an insertion indicator.
'quarry' becomes 'mine'.
'ag' placed within 'mine' is 'magine'.
'i'+'magine'='IMAGINE'
(Other definitions for imagine that I've seen before include "See something that doesn't exist" , "Fancy - Lennon song" , "Picture mentally" , "Picture to oneself" , "Suspect" .)