Gratuity in post used as wage (7)
I believe the answer is:
stipend
'used as wage' is the definition.
(a stipend may form a salary)
'gratuity in post' is the wordplay.
'gratuity' becomes 'tip' ('tip' can be a synonym of 'gratuity').
'in' indicates putting letters inside.
'post' becomes 'send' ('send' can be a synonym of 'post'**).
'tip' put into 'send' is 'STIPEND'.
(Other definitions for stipend that I've seen before include "Soldier's pay" , "Tips end badly in an emolument" , "Church pay" , "Pay (clerical)" , "Clergyman's salary" .)