Coins numismatist necessarily holds up (5)
I believe the answer is:
cents
'coins' is the definition.
(cent is a kind of coin)
'numismatist necessarily holds up' is the wordplay.
'holds' says the answer is hidden in the clue.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'stnec' is hidden within 'numismatist necessarily'.
'stnec' in reverse letter order is 'CENTS'.
(Other definitions for cents that I've seen before include "Hundredths of dollar" , "US money, e.g." , "money abroad" , "American coins" , "Fractions of dollar" .)