Where Luke is found, or Frank (8)

I believe the answer is:

postmark

I'm a little stuck... Click here to teach me more about this clue!

'frank' is the definition.
(synonyms)

I don't understand the rest of the clue.

(Other definitions for postmark that I've seen before include "It shows date and time of mailing" , "Letter franking" , "Stamp franking" , "It is stamped on envelope" , "Frank" .)

Want a hint initially instead of a full solution? Install my app