Huge number sick in heaven (7)
I believe the answer is:
zillion
'huge number' is the definition.
(I've seen this before)
'sick in heaven' is the wordplay.
'sick' becomes 'ill' (similar in meaning).
'in' is an insertion indicator.
'heaven' becomes 'zion' (I've seen this in another clue).
'ill' going into 'zion' is 'ZILLION'.
(Other definitions for zillion that I've seen before include "Many millions" , "Indefinite multiple of 22 Across" , "Mind-blowing number" , "Huge, unspecified figure" , "Extremely large but unspecified number" .)