Lock up silver in church (4)
I believe the answer is:
cage
'lock up' is the definition.
(I've seen this in another clue)
'silver in church' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'in' means one lot of letters goes inside another.
'church' becomes 'CE' (abbreviation for Church of England).
'ag' placed within 'ce' is 'CAGE'.
(Other definitions for cage that I've seen before include "Enclosure made from metal bars" , "Structure of bars or wire" , "Composer working in silence" , "Pen" , "Barred structure" .)