Fancy silver found in one mine (7)
I believe the answer is:
imagine
'fancy' is the definition.
(I know that fancy is a more specific form of the action imagine)
'silver found in one mine' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'found in' means one lot of letters goes inside another.
'one' becomes 'I' (Roman numeral).
'i'+'mine'='imine'
'ag' placed inside 'imine' is 'IMAGINE'.
(Other definitions for imagine that I've seen before include "A Gemini? (anag)" , "John Lennon hit" , "Form a mental image in the mind" , "Reckon" , "Conceive in the mind" .)