Picture silver in island repository (7)
I believe the answer is:
imagine
'picture' is the definition.
(picture can mean to visualise or imagine)
'silver in island repository' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'in' is an insertion indicator.
'island repository' becomes 'imine' (I am not sure about this - if you are sure you should believe this answer much more).
'ag' going into 'imine' is 'IMAGINE'.
Can you help me to learn more?
(Other definitions for imagine that I've seen before include "Picture, conceive" , "Assume" , "Conceive in the mind" , "1971 John Lennon song" , "Picture mentally" .)