Researcher's mail counter paid for on delivery (7)
I believe the answer is:
postdoc
'researcher's' is the definition.
(I've seen this before)
'mail counter paid for on delivery' is the wordplay.
'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' written backwards gives 'doc'.
'post'+'doc'='POSTDOC'
(Other definitions for postdoc that I've seen before include "Someone doing advanced research" , "Researcher" .)