Not as much to be thankful for having lost billions (4)
I believe the answer is:
less
'not as much' is the definition.
(similar in meaning)
'be thankful for having lost billions' is the wordplay.
'be thankful for' becomes 'bless' (I've seen this before).
'having lost' indicates named letters should be taken away.
'billions' becomes 'b' (abbreviation).
'bless' with 'b' taken out is 'LESS'.
'to' is the link.
(Other definitions for less that I've seen before include "In smaller quantity" , "more, as they say" , "Minus; not so much" , "Fewer; minus" , "inferior" .)