Observe some paper money (4)
I believe the answer is:
note
This clue is a triple definition!
'observe' is the first definition.
(I know that observe can be written as note)
'some paper' is the second definition.
(thesaurus)
'money' is the third definition.
(thesaurus)
(Other definitions for note that I've seen before include "It could be me" , "Observe in Eton" , "Fiver, say" , "Tenner, for example" , "Crotchet or minim, perhaps" .)