Mark to pause, initially carrying bullets northwards (5)
I believe the answer is:
comma
'mark to pause' is the definition.
(punctuation mark indicating a pause)
'initially carrying bullets northwards' is the wordplay.
'initially' indicates taking the first letters.
'bullets' becomes 'ammo' (I've seen this before).
'northwards' says the letters should be written in reverse (in a down clue, letters go up or 'north').
The first letter of 'carrying' is 'c'.
'ammo' in reverse letter order is 'omma'.
'c'+'omma'='COMMA'
(Other definitions for comma that I've seen before include ", which gives us pause" , "Punctuation mark indicating slight pause" , "Puncuation mark" , "its use may be divisive" , "Brief stop" .)