Failing old firm in security almost unknown (10)
I believe the answer is:
deficiency
'failing' is the definition.
(thesaurus)
'old firm in security almost unknown' is the wordplay.
I cannot quite understand how this works, but
'unknown' could be 'y' (algebra) and 'y' is located in the answer.
'old firm' could be 'ici' (former company Imperial Chemical Industries) and 'ici' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for deficiency that I've seen before include "Lack - want" , "Shortcoming" , "Shortage" .)