Salary? Mine must be returned in post! (7)
I believe the answer is:
Here is my best explanation:
'salary?' is the definition.
'must be returned in post' is the subsidiary indication.
'must' becomes 'pit' (I can't explain this - if you can you should believe this answer much more).
'be returned' says the letters should be written backwards.
'in' means one lot of letters goes inside another.
'post' becomes 'send' ('send' can be a synonym of 'post').
'pit' in reverse letter order is 'tip'.
'tip' inserted inside 'send' is 'stipend'.
'mine' is the link.
This may not be correct. It may belong to another bit of the clue.
(Other definitions for stipend that I've seen before include "Regular payment", "A salary", "Payment for cleric", "Periodical allowance", "regular payment", "Pay (clerical)", "Regular money", "Official income of Protestant clergyman", "minister's wages" and "Fixed salary".)