Scrounge a large one (6)
I believe the answer is:
bumper
'large one' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?
'scrounge a' is the wordplay.
'scrounge' becomes 'bum' (I can't justify this - if you can you should believe this answer much more).
'a' becomes 'per' (eg 'one a day' means 'one per day').
'bum'+'per'='BUMPER'
Can you help me to learn more?
(Other definitions for bumper that I've seen before include "Protective vehicle bar" , "Glass filled to brim" , "Part of a car" , "Exceptionally large" , "Very large - vehicle protector" .)