Researcher's mail counter paid for on delivery (7)
I believe the answer is:
Here is my best explanation:
'researcher's' is the definition.
(I've seen this before)
'mail counter paid for on delivery' is the subsidiary indication.
'mail' becomes 'post' (I've seen this before).
'counter' shows that the letters should be reversed in order.
'paid for on delivery' becomes 'cod' (abbreviation for 'cash on delivery').
'cod' backwards is 'doc'.
(Other definitions for postdoc that I've seen before include "Researcher" and "Someone doing advanced research".)