Allowance: gratuity put in post (7)
I believe the answer is:
stipend
'allowance' is the definition.
(I've seen this before)
'gratuity put in post' is the wordplay.
'gratuity' becomes 'tip' (synonyms).
'put in' means one lot of letters goes inside another.
'post' becomes 'send' (synonyms).
'tip' inserted within 'send' is 'STIPEND'.
(Other definitions for stipend that I've seen before include "Tips end badly in an emolument" , "Church pay" , "Fixed salary" , "Salary, allowance" , "Wage" .)