Unspecified multitude laid up in the heavenly city? (7)
I believe the answer is:
zillion
'unspecified multitude' is the definition.
'laid up in the heavenly city?' is the wordplay.
'laid up' becomes 'ill' ('ill' can be similar in meaning to 'laid up').
'in the' indicates putting letters inside.
'heavenly city?' becomes 'zion'.
'ill' going within 'zion' is 'ZILLION'.
(Other definitions for zillion that I've seen before include "Indefinite large number, lots of noughts" , "Great quantity" , "Many millions" , "Unspecified enormous number" , "Unimaginably high number" .)