Regular payment in post includes gratuity (7)
I believe the answer is:
stipend
'regular payment' is the definition.
(stipend is a kind of regular payment)
'post includes gratuity' is the wordplay.
'post' becomes 'send' ('send' can be a synonym of 'post'**).
'includes' is an insertion indicator.
'gratuity' becomes 'tip' ('tip' can be a synonym of 'gratuity').
'send' placed around 'tip' is 'STIPEND'.
'in' is the link.
(Other definitions for stipend that I've seen before include "Minister's salary" , "minister's wages" , "Pay (clerical)" , "Soldier's pay" , "Parson's pay" .)