Believe Bill has surplus wealth (8)
I believe the answer is:
accredit
'believe' is the definition.
(I've seen this in another clue)
'bill has surplus wealth' is the wordplay.
'bill' becomes 'ac' (short for account).
'has' is a charade indicator (letters next to each other).
'surplus wealth' becomes 'credit' (I can't explain this - if you can you should believe this answer much more).
'ac'+'credit'='ACCREDIT'
Can you help me to learn more?
(Other definitions for accredit that I've seen before include "Accept as true" , "Certify, approve" , "Officially recognise" , "Attribute (something to)" , "Certify, ascribe" .)