Very large unspecified number not well in Jerusalem (7)
I believe the answer is:
zillion
'very large unspecified number' is the definition.
(I've seen this before)
'not well in jerusalem' is the wordplay.
'not well' becomes 'ill' ('ill' is the opposite to 'well').
'in' means one lot of letters goes inside another.
'jerusalem' becomes 'zion' (a name for Jerusalem in the Hebrew Bible).
'ill' placed into 'zion' is 'ZILLION'.
(Other definitions for zillion that I've seen before include "Unimaginably high number" , "Huge, unspecified figure" , "Great quantity" , "Extremely large but unspecified number" , "Many millions" .)