Buck model artist up (6)
I believe the answer is:
dollar
'buck' is the definition.
(slang term for a dollar)
'model artist up' is the wordplay.
'model' becomes 'doll' (a doll is a model of a person).
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'ra' reversed gives 'ar'.
'doll'+'ar'='DOLLAR'
(Other definitions for dollar that I've seen before include "The almighty currency, a buck" , "Greenback; buck" , "Philip -, English poet" , "American currency" , "Economic factors deployed as 'soft power'" .)