Salary? Mine must be returned in post! (7)
I believe the answer is:
stipend
'salary?' is the definition.
(thesaurus)
'be returned in post' is the wordplay.
'be returned' becomes 'tip' (I am not sure about this - if you are sure you should believe this answer much more).
'in' is an insertion indicator.
'post' becomes 'send' ('send' can be a synonym of 'post').
'tip' placed within 'send' is 'STIPEND'.
'mine must' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for stipend that I've seen before include "Regular payment" , "Official income of Protestant clergyman" , "Fixed salary" , "Regular money" , "Periodical allowance" .)