Act on electronic malfunction without delay (2,4)
I believe the answer is:
at once
'without delay' is the definition.
(I've seen this in another clue)
'act on electronic malfunction' is the wordplay.
'electronic' becomes 'e' (as in e-mail or e-books).
'malfunction' indicates an anagram.
'act'+'on'+'e'='actone'
'actone' anagrammed gives 'AT ONCE'.
(Other definitions for at once that I've seen before include "This instant, without delay" , "from the kick-off" , "without hesitation" , "Immediately, without delay" , "Immediately, this instant" .)