A bushel and a pound for a tuber (4)
I believe the answer is:
bulb
'tuber' is the definition.
(thesaurus)
'a bushel and a pound' is the wordplay.
'a bushel' becomes 'bu' (I can't justify this - if you can you should give a lot more credence to this answer).
'and' is a charade indicator (letters next to each other).
'a pound' becomes 'lb' (abbreviation for the unit of weight).
'bu'+'lb'='BULB'
'for a' is the link.
Can you help me to learn more?
(Other definitions for bulb that I've seen before include "From which a tulip or light comes" , "shiner" , "This provides light" , "Eg, hyacinth" , "Flower seed or lighting device" .)