Put it in the post, to be courteous (6)
I believe the answer is:
polite
'be courteous' is the definition.
(similar in meaning)
'put it in the post' is the wordplay.
'put' indicates putting letters inside.
'in the post' becomes 'pole' (I can't explain this - if you can you should give a lot more credence to this answer).
'it' put within 'pole' is 'POLITE'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for polite that I've seen before include "Well-behaved" , "Well-mannered, courteous" , "Mannerly, courteous" , "Respectful and considerate" , "Good mannered and civil" .)