Over £501 billion in account? (4)
I believe the answer is:
bill
'account?' is the definition.
(I know that account can be written as bill)
'over ps501 billion' is the wordplay.
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'ps50i' becomes 'ill' (I can't explain this - if you can you should give a lot more credence to this answer).
'billion' becomes 'b'.
'ill' put after 'b' is 'BILL'.
'in' is the link.
Can you help me to learn more?
(Other definitions for bill that I've seen before include "beak demanded" , "Account - used for cutting and pruning" , "Poster; draft law" , "Statute in draft before it becomes law" , "Invoice; man's name" .)