Being careless is responsible for United getting relegated (6)
I believe the answer is:
casual
'being careless' is the definition.
('casual' can be similar in meaning to 'careless')
I cannot understand how the remainder of the clue works.
(Other definitions for casual that I've seen before include "Free-and-easy" , "Relaxed; informal" , "Careless; unsystematic" , "Irregular" , "Informal or cursory" .)