A current hack shown up in old computers (5)
I believe the answer is:
abaci
'old computers' is the definition.
(an abacus is an ancient device for computation)
'a current hack shown up' is the wordplay.
'current' becomes 'I' (symbol for electric current).
'hack' becomes 'cab' (synonyms).
'shown up' says the letters should be written in reverse (in a down clue, letters go up).
'i'+'cab'='icab'
'icab' reversed gives 'baci'.
'a'+'baci'='ABACI'
'in' acts as a link.
(Other definitions for abaci that I've seen before include "frames" , "They can be counted on" , "on which people may be counting?" , "summers" , "Counters" .)