Clerics put uranium in boxes (7)
I believe the answer is:
curates
'clerics' is the definition.
(I've seen this before)
'uranium in boxes' is the wordplay.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'in' is an insertion indicator.
'boxes' becomes 'crates' (crate is a kind of box).
'u' going within 'crates' is 'CURATES'.
'put' is the link.
(Other definitions for curates that I've seen before include "Members of the clergy" , "Clerics" , "As ministry assistants" , "Vicars' seconds" , "Servicemen" .)