Broken-down company line no longer valid (9)
I believe the answer is:
collapsed
'broken-down' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I don't see?
'company line no longer valid' is the wordplay.
'company' becomes 'co' (abbreviation).
'line' becomes 'l' (used when specifying particular lines from a poem).
'no longer valid' becomes 'lapsed' (I am not sure about this - if you are sure you should believe this answer much more).
'co'+'l'+'lapsed'='COLLAPSED'
Can you help me to learn more?
(Other definitions for collapsed that I've seen before include "laid low" , "Gave way, caved in" , "Fell apart" , "Cracked up - gave way" .)