Visualise one pit containing silver (7)
I believe the answer is:
imagine
'visualise' is the definition.
(I know this)
'one pit containing silver' is the wordplay.
'one' becomes 'I' (Roman numeral).
'pit' becomes 'mine' (mining pit).
'containing' means one lot of letters goes inside another.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'mine' placed around 'ag' is 'magine'.
'i'+'magine'='IMAGINE'
(Other definitions for imagine that I've seen before include "that Lennon composed" , "A Gemini? (anag)" , "Assume" , "Suppose; think up" , "Conjure up in mind, visualise" .)