Nut is an extremely polite put down (5)
I believe the answer is:
pecan
'nut' is the definition.
(type of nut)
'an extremely polite put down' is the wordplay.
'an extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'put down' becomes 'can' (I can't explain this - if you can you should give a lot more credence to this answer).
'polite' with its middle taken out is 'pe'.
'pe'+'can'='PECAN'
'is' is the link.
Can you help me to learn more?
(Other definitions for pecan that I've seen before include "Something like a walnut" , "Nut of American hickory tree" , "Type of nut" , "US nut tree" , "Smooth pinkish-brown nut, nice in pie" .)