Regular payment in post, including gratuity (7)
I believe the answer is:
stipend
'regular payment' is the definition.
(stipend is a kind of regular payment)
'post including gratuity' is the wordplay.
'post' becomes 'send' ('send' can be a synonym of 'post'**).
'including' indicates putting letters inside.
'gratuity' becomes 'tip' ('tip' can be a synonym of 'gratuity').
'send' enclosing 'tip' is 'STIPEND'.
'in' is the link.
(Other definitions for stipend that I've seen before include "Official income of Protestant clergyman" , "Salary, allowance" , "Minister's salary" , "Periodical allowance" , "Regular money" .)