Nothing works — sorry about that! (4)
I believe the answer is:
oops
'sorry about that' is the definition.
('oops' is said to acknowledge a mistake)
'nothing works' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'works' becomes 'ops' (op is short for opus, a work).
'o'+'ops'='OOPS'
(Other definitions for oops that I've seen before include "Exclamation recognising a mistake has been made" , "Exclamation on making minor error" , "Oh dear, that's a mistake!" , "I'm sorry" , "Harem" .)