Fixed allowance and gratuity in post (7)
I believe the answer is:
stipend
'fixed allowance' is the definition.
'gratuity in post' is the wordplay.
'gratuity' becomes 'tip' ('tip' can be a synonym of 'gratuity').
'in' means one lot of letters goes inside another.
'post' becomes 'send' ('send' can be a synonym of 'post'**).
'tip' put within 'send' is 'STIPEND'.
'and' is the link.
(Other definitions for stipend that I've seen before include "Clerical salary" , "Minister's salary" , "Regular money" , "Regular payment" , "Clergyman's salary" .)