Pay bonus in post (7)
I believe the answer is:
stipend
'pay' is the definition.
(I've seen this before)
'bonus in post' is the wordplay.
'bonus' becomes 'tip' (I've seen this before).
'in' indicates putting letters inside.
'post' becomes 'send' (synonyms).
'tip' inserted within 'send' is 'STIPEND'.
(Other definitions for stipend that I've seen before include "Periodical allowance" , "Pay (clerical)" , "Soldier's pay" , "Church pay" , "Regular money" .)