Shoots up with an addition to a letter (5)
I believe the answer is:
serif
'an addition to a letter' is the definition.
(I've seen this in another clue)
'shoots up' is the wordplay.
'shoots' becomes 'fires' (firing is a kind of shooting).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'fires' reversed gives 'SERIF'.
'with' is the link.
(Other definitions for serif that I've seen before include "Brazil port" , "Letter feature" , "Small line used in printing type" , "Projection at a letter's end" , "Slight projection finishing off printed letter - puts fires out" .)