Stamp on a stamp (8)
I believe the answer is:
postmark
This could be a double definition.
'stamp on' is the first definition.
'postmark' can be an answer for 'stamp' (postmarking is a kind of stamping). I am not certain of the 'on' bit.
'a stamp' is the second definition.
(I know that postmark is a more specific form of the action stamp)
Can you help me to learn more?
(Other definitions for postmark that I've seen before include "Stamp cancellation" , "It is stamped on envelope" , "Letter franking" , "It shows date and time of mailing" , "Frank" .)