Not working is unacceptable (3,2,5)
I believe the answer is:
out of order
This clue is a double definition.
'not working' is the first definition.
(I've seen this before)
'unacceptable' is the second definition.
(I've seen this before)
'is' acts as a link.
(Other definitions for out of order that I've seen before include "unusually arranged" , "having left monastery?" , "(Mechanism) broken" , "Ex-nun is" , "Disgraced knight might be told to get this" .)