Breakdown of French cable system (7)
I believe the answer is:
debacle
'breakdown' is the definition.
(debacle is a kind of breakdown)
'of french cable system' is the wordplay.
'of french' becomes 'de' ('of' in French).
'system' indicates an anagram (another version or system with the same letters).
'cable' is an anagram of 'bacle'.
'de'+'bacle'='DEBACLE'
(Other definitions for debacle that I've seen before include "A sound defeat" , "Humiliating failure" , "A complete collapse or messy failure" , "Utter disaster" , "Sudden and violent collapse" .)