Weakness of one former car manufacturer assimilated by rival (6)
I believe the answer is:
foible
'weakness' is the definition.
(thesaurus)
'one former car manufacturer assimilated by rival' is the wordplay.
'one' becomes 'i' (Roman numeral).
'former car manufacturer' becomes 'bl' (British Leyland).
'assimilated by' is an insertion indicator.
'rival' becomes 'foe' (foe is a kind of rival).
'i'+'bl'='ibl'
'ibl' put inside 'foe' is 'FOIBLE'.
'of' acts as a link.
(Other definitions for foible that I've seen before include "Minor eccentricity" , "weakness" , "'Mannerism, idiosyncrasy (6)'" , "Quirk" , "Harmless peculiarity in person's character" .)