Payment and a bit extra in post (7)
I believe the answer is:
stipend
'payment' is the definition.
(stipend is a kind of payment)
'a bit extra in post' is the wordplay.
'a bit extra' becomes 'tip' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'post' becomes 'send' (synonyms).
'tip' inserted inside 'send' is 'STIPEND'.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for stipend that I've seen before include "Salary, allowance" , "Wage" , "Clerical salary" , "Tips end badly in an emolument" , "Regular money" .)