Coins numismatist necessarily holds up (5)
I believe the answer is:
Here is my best explanation:
'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 "money abroad", "American coins", "US money, e.g.", "hundredths", "Hundredths of dollar", "fractions" and "Fractions of dollar".)