Observe some paper money (4)
I believe the answer is:
note
I believe this clue is a triple definition!
'observe' is the first definition.
(synonyms)
'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" .)