Impressions of a country's postal service (6)
I believe the answer is:
stamps
'impressions' is the definition.
(I know this)
'a country's postal service' is the wordplay.
I cannot quite see how this works, but
'a' could be 'amp' (A is the symbol for the amp or ampere) and 'amp' is located in the answer.
The remaining letters 'sts' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for stamps that I've seen before include "Puts the foot down on letters" , "Cuts out with a die" , "Items of interest to a philatelist" , "Philatelists study them" , "Brings foot heavily to the ground, as in anger" .)