Popular Greek character conceals money (5)
I believe the answer is:
rhino
'money' is the definition.
(old-fashioned slang term for money)
'popular greek character conceals' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'greek character' becomes 'rho' (letter from the Greek alphabet).
'conceals' is an insertion indicator.
'in' placed within 'rho' is 'RHINO'.
(Other definitions for rhino that I've seen before include "The necessary" , "One with a very thick skin (abbr)" , "Funds" , "one might charge in Africa" , "old-fashioned bread" .)