Secure silver in church (4)
I believe the answer is:
cage
'secure' is the definition.
'silver in church' is the wordplay.
'silver' becomes 'ag' ('Ag' can be a synonym of 'silver').
'in' means one lot of letters goes inside another.
'church' becomes 'ce' (abbreviation for Church of England).
'ag' put inside 'ce' is 'CAGE'.
(Other definitions for cage that I've seen before include "Confine -- US composer, d. 1992" , "Barred structure" , "Structure of bars or wire" , "Nicolas - - (films)" , "Incarcerate" .)