Cover wager about large bank losing billions (7)
I believe the answer is:
blanket
'cover' is the definition.
(I know that cover can be written as blanket)
'wager about large bank losing billions' is the wordplay.
'wager' becomes 'bet' (synonyms).
'about' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'losing' suggests deleting specific letters.
'billions' becomes 'b' (abbreviation).
'l'+'bank'='lbank'
'lbank' with 'b' removed is 'lank'.
'bet' going around 'lank' is 'BLANKET'.
(Other definitions for blanket that I've seen before include "Thick item on bed" , "coverage" , "Comprehensive" , "Bed cover; inclusive" , "Thick bed cover" .)