Something added to letters in sacks sent north (5)
I believe the answer is:
serif
'north' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for serif that I've seen before include "Typeface decoration" , "Detail on some typefaces" , "Feature in some fonts" , "Projection at a letter's end" , "Brazil port" .)