Pound sterling, informally (4)
I believe the answer is:
quid
'pound sterling informally' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for quid that I've seen before include "PS (sl.)" , "Pound sterling (slang)" , "bit of bread" , "Wad of chewing tobacco" , "One fifth of a fiver" .)